Several years ago Quilter's Newsletter magazine ask this same question. Several quilters responded, me included.
In a later issue they discussed the same subject and mine was the only answer they gave......MY head is big this morning. So here it is. This is based on messing up the squares many times and designing the preprinted whole cloth quilts.
Place a mark around the outside edge of the area you want to grid. Use 1" or whatever measurement you want. a 1" on the diagonal will give you about a 3/4" square in the grid.
Then connect the dots from side to side. You can use whatever method you want to mark the line. However, In find that even if I don't mark the entire length of the line, placing tick marks down the row helps me to keep the squares even. I tend, however to mark the entire line. I have had too many squares turn into rectangles or womply diamonds when I don't mark for alighnment