P3 — Basic grid based on size

grid.js

void setup() {
  size(800,800);
}

void draw() {

int tilesX = 10;
int tilesY = 1;

int tileW = int(width/tilesX);
int tileH = int(height/tilesY);

for (int x = 0; x < width; x += tileW) {
  for (int y = 0; y < height; y+= tileH) {
      stroke(255);
      fill(0);
      rect(x, y, tileW, tileH);
    }
  }
}