ArmorBaseItem.java

package armor;

import net.minecraft.client.renderer.entity.model.BipedModel;
import net.minecraft.entity.Entity;
import net.minecraft.entity.LivingEntity;
import net.minecraft.inventory.EquipmentSlotType;
import net.minecraft.item.ArmorItem;
import net.minecraft.item.IArmorMaterial;
import net.minecraft.item.Item;
import net.minecraft.item.ItemStack;

public class ArmorBaseItem extends ArmorItem {

    private Supplier<Supplier<ArmorBaseModel>> armorModel;

    public ArmorBaseItem(IArmorMaterial material, EquipmentSlotType equipmentSlot, Item.Properties properties, Supplier<Supplier<ArmorBaseModel>> armorModel) {
        super(material, equipmentSlot, properties);
        this.armorModel = armorModel;
    }
    
    @OnlyIn(Dist.CLIENT)
    @Nullable
    @Override
    public final BipedModel getArmorModel(LivingEntity entity, ItemStack itemStack, EquipmentSlotType armorSlot, BipedModel defaultArmor) {
        return armorModel.get().get().applyEntityStats(defaultArmor).applySlot(armorSlot);
    }

    @Nullable
    @Override
    public final String getArmorTexture(ItemStack stack, Entity entity, EquipmentSlotType slot, String type) {
        return armorModel.get().get().getTexture();
    }
}

ArmorBaseModel.java

package armor;

import com.mojang.blaze3d.matrix.MatrixStack;
import com.mojang.blaze3d.vertex.IVertexBuilder;
import net.minecraft.client.renderer.entity.model.BipedModel;
import net.minecraft.client.renderer.model.ModelRenderer;
import net.minecraft.inventory.EquipmentSlotType;
import net.minecraft.util.ResourceLocation;

public abstract class ArmorBaseModel extends BipedModel<LivingEntity> {
    protected final ModelRenderer armorHead;
    protected final ModelRenderer armorBody;
    protected final ModelRenderer armorRightArm;
    protected final ModelRenderer armorLeftArm;
    protected final ModelRenderer armorRightLeg;
    protected final ModelRenderer armorLeftLeg;
    protected final ModelRenderer armorRightBoot;
    protected final ModelRenderer armorLeftBoot;

    private String texture;

    public ArmorBaseModel(int textureWidth, int textureHeight, ResourceLocation texture){
        super(1F);
        this.textureWidth = textureWidth;
        this.textureHeight = textureHeight;
        this.texture = texture.toString();

        armorHead = new ModelRenderer(this);
        armorHead.setRotationPoint(0.0F, 0.0F, 0.0F);
        bipedHead.addChild(armorHead);

        armorBody = new ModelRenderer(this);
        armorBody.setRotationPoint(0.0F, 0.0F, 0.0F);
        bipedBody.addChild(armorBody);
        armorRightArm = new ModelRenderer(this);
        armorRightArm.setRotationPoint(0.0F, 0.0F, 0.0F);
        bipedRightArm.addChild(armorRightArm);

        armorLeftArm = new ModelRenderer(this);
        armorLeftArm.setRotationPoint(0.0F, 0.0F, 0.0F);
        bipedLeftArm.addChild(armorLeftArm);

        armorRightLeg = new ModelRenderer(this);
        armorRightLeg.setRotationPoint(0.0F, 0.0F, 0.0F);
        bipedRightLeg.addChild(armorRightLeg);

        armorLeftLeg = new ModelRenderer(this);
        armorLeftLeg.setRotationPoint(0.0F, 0.0F, 0.0F);
        bipedLeftLeg.addChild(armorLeftLeg);


        armorRightBoot = new ModelRenderer(this);
        armorRightBoot.setRotationPoint(0.0F, 0.0F, 0.0F);
        bipedRightLeg.addChild(armorRightBoot);

        armorLeftBoot = new ModelRenderer(this);
        armorLeftBoot.setRotationPoint(0.0F, 0.0F, 0.0F);
        bipedLeftLeg.addChild(armorLeftBoot);

        setupArmorParts();
    }

    public abstract void setupArmorParts();

    public final String getTexture(){
        return this.texture;
    }

    /**
     * Feel free to override this method as needed.
     * It's just required to hide armor parts depending on the equipment slot
     */
    public BipedModel applySlot(EquipmentSlotType slot){
        armorHead.showModel = false;
        armorBody.showModel = false;
        armorRightArm.showModel = false;
        armorLeftArm.showModel = false;
        armorRightLeg.showModel = false;
        armorLeftLeg.showModel = false;
        armorRightBoot.showModel = false;
        armorLeftBoot.showModel = false;

        switch(slot){
            case HEAD:
                armorHead.showModel = true;
                break;
            case CHEST:
                armorBody.showModel = true;
                armorRightArm.showModel = true;
                armorLeftArm.showModel = true;
                break;
            case LEGS:
                armorRightLeg.showModel = true;
                armorLeftLeg.showModel = true;
                break;
            case FEET:
                armorRightBoot.showModel = true;
                armorLeftBoot.showModel = true;
                break;
            default:
                break;
        }

        return this;
    }

    public final ArmorBaseModel applyEntityStats(BipedModel defaultArmor){
        this.isChild = defaultArmor.isChild;
        this.isSneak = defaultArmor.isSneak;
        this.isSitting = defaultArmor.isSitting;
        this.rightArmPose = defaultArmor.rightArmPose;
        this.leftArmPose = defaultArmor.leftArmPose;

        return this;
    }

    @Override
    public void render(MatrixStack matrixStack, IVertexBuilder buffer, int packedLight, int packedOverlay, float red, float green, float blue, float alpha) {
        copyModelAngles(this.bipedHead, this.armorHead);
        copyModelAngles(this.bipedBody, this.armorBody);
        copyModelAngles(this.bipedRightArm, this.armorRightArm);
        copyModelAngles(this.bipedLeftArm, this.armorLeftArm);
        copyModelAngles(this.bipedRightLeg, this.armorRightLeg);
        copyModelAngles(this.bipedLeftLeg, this.armorLeftLeg);
        copyModelAngles(this.bipedRightLeg, this.armorRightBoot);
        copyModelAngles(this.bipedLeftLeg, this.armorLeftBoot);

        matrixStack.push();
        if(isSneak) matrixStack.translate(0, 0.2, 0);

        renderChildrenOnly(this.bipedHead, matrixStack, buffer, packedLight, packedOverlay, red, green, blue, alpha);
        renderChildrenOnly(this.bipedBody, matrixStack, buffer, packedLight, packedOverlay, red, green, blue, alpha);
        renderChildrenOnly(this.bipedRightArm, matrixStack, buffer, packedLight, packedOverlay, red, green, blue, alpha);
        renderChildrenOnly(this.bipedLeftArm, matrixStack, buffer, packedLight, packedOverlay, red, green, blue, alpha);
        renderChildrenOnly(this.bipedRightLeg, matrixStack, buffer, packedLight, packedOverlay, red, green, blue, alpha);
        renderChildrenOnly(this.bipedLeftLeg, matrixStack, buffer, packedLight, packedOverlay, red, green, blue, alpha);

        matrixStack.pop();
    }

    public final void setRotationAngle(ModelRenderer modelRenderer, float x, float y, float z) {
        modelRenderer.rotateAngleX = x;
        modelRenderer.rotateAngleY = y;
        modelRenderer.rotateAngleZ = z;
    }

    private final void copyModelAngles(ModelRenderer in, ModelRenderer out){
        out.rotateAngleX = in.rotateAngleX;
        out.rotateAngleY = in.rotateAngleY;
        out.rotateAngleZ = in.rotateAngleZ;
    }
    
    private final void renderChildrenOnly(ModelRenderer bodyPart, MatrixStack matrixStack, IVertexBuilder buffer, int packedLight, int packedOverlay, float red, float green, float blue, float alpha){
        if(!bodyPart.childModels.isEmpty()){
            matrixStack.push();
            bodyPart.translateRotate(matrixStack);
            for(ModelRenderer child : bodyPart.childModels)
                child.render(matrixStack, buffer, packedLight, packedOverlay, red, green, blue, alpha);
            matrixStack.pop();
        }
    }
}

ArmorModelImplementation.java

public class ArmorModelImplementation extends ArmorBaseModel {

	public ArmorModelImplementation() {
		super(/*texture width*/, /*texture height*/, new ResourceLocation(/*Texture path*/));
	}

	@Override
	public void setupArmorParts() {
    //You need to cut out some parts from Blockbenchs exported model, and paste them here.
    //You can see 2 examples of how this may look, adding a cube to the head
    //and a rotated cube on the chest
    //Export the unedited armor template to see which parts you should **not** copy over
		armorHead.cubeList.add(new ModelBox(armorHead, 0, 18, -0.5F, -22.0F, -5.0F, 1, 22, 10, 0.0F, false));
    
		ModelRenderer bone = new ModelRenderer(this);
		bone.setRotationPoint(0.0F, 0.0F, 0.0F);
		setRotationAngle(bone, 0.0F, 0.0F, -0.7854F);
		armorBody.addChild(bone);
		bone.cubeList.add(new ModelBox(bone, 18, 82, -4.0F, 1.0F, -3.0F, 3, 3, 1, 0.0F, false));
  }
}

ItemRegistrationExample.java

//This needs to be it's own class, I don't recommend doing anything else in here!
//Otherwise you risk being incompatible with servers.
public class ClientArmorHelper{

  private final static ArmorBaseModel YOUR_FIRST_ARMOR = new ArmorModelImplementation();
  private final static ArmorBaseModel YOUR_SECOND_ARMOR = new SomeOtherArmorModelImplementation();

  public static ArmorBaseModel getFirstArmor(){
    return YOUR_FIRST_ARMOR;
  }
  
  public static ArmorBaseModel getSecondArmor(){
    return YOUR_SECOND_ARMOR;
  }
}

//Creating an Armor Item now looks similar to this:
new ArmorBaseItem(material, slotType, properties, () -> ClientArmorHelper::getFirstArmor);