Blog post - http://www.jitendrazaa.com/blog/salesforce/dynamically-instantiate-and-destroy-lightning-components-modal-dialog-component/

ModalWindow.cmp

<aura:component > 
    <aura:attribute name="title" type="String" required="true"/>
     
    
     <div class="demo-only"  style="height: 100%;">
            <section  class="slds-modal slds-fade-in-open">
                <div class="slds-modal__container">
                    <header class="slds-modal__header">
                        <button class="slds-button slds-modal__close" title="Close" 
                                onclick="{!c.closeMe}">
                            <lightning:icon class="white slds-icon_small" iconName="utility:close"/> 
                            <span class="slds-assistive-text">Close</span>
                        </button>
                        <h2 id="modal-heading-01" class="slds-text-heading_medium slds-hyphenate">
                            {#v.title}</h2>
                    </header>
                    <div class="slds-modal__content slds-p-around_medium" > 
                        {#v.body}
                    </div>
                    <footer class="slds-modal__footer"> 
                        <button class="slds-button slds-button_brand" onclick="{!c.closeMe}">Ok</button>
                    </footer>
                </div>
            </section>
            <div class="slds-backdrop slds-backdrop_open"></div>
        </div>
</aura:component>