Place the photo underneath the acetate and align a corner. Place a piece of tape halfway on the photo securely attaching it to the acetate.
Cut the acetate to match the dimensions of your photo. Be precise since this will be your guide for the painting.
Measure the acetate you just cut by length. Round the number down to the nearest inch—if it is 12 and 3/4 inches, it becomes 12 inches. Measure the width and round the number down to the nearest inch. Record the dimensions. Subtract the rounded numbers from the total distances; these are your “allowance” measurements for the grid.
Separate the picture from the acetate so that it does not get marked. Starting with the length, find the beginning of where you want to start your reference point on the acetate (you may need to check alignments again with the photo, but be sure to separate both again before marking) and mark it with a visible dot; you can start after the allowance measurement or save it for the end of the line, but your starting point must correspond with the opposite side so it stays even.
Mark each inch until you get to your rounded measurement for that side. Mark each inch on the opposite side until both sides have corresponding dots. Using the ruler, connect the opposite corresponding points.
Mark the width in the same way as Step 4. After you finish marking the opposite side, connect the corresponding dots with the ruler. Let the ink dry, then tape the photo to the back of the acetate. You should see a perfect grid on top of your photo.
Measure the length and width of your canvas. Divide the width and length of the canvas separately with the rounded width and length of your grid. Round the results down to the nearest inch to give yourself free space on the edges of your canvas.
Multiply these numbers with the corresponding rounded length and width of the grid to get the rounded width and length of the canvas. Divide these numbers with the rounded length and width of the canvas to get the measurements for each square.
Measuring with the ruler, mark out each point carefully with the watercolor pencil, first lengthwise and then on the opposite side. Connect them using the ruler's straight-edge. Plot out each point carefully for the width and do the same for its opposite side. You now have your gridded canvas.