I like to use "Scotch-Blue Painter's Tape" to insure straight lines when I quilt a grid pattern. It doesn't pull on my quilt when I remove it, and I can re-use the same strip, and the bright color makes it easy to follow.
Lay your quilt on a flat surface, and lay a strip of tape in the desired direction along the middle of your quilt. Sew along both sides, being careful to not puncture the tape. Working your way towards the edges of your quilt, pick up the tape, reposition and sew.
I used the 2 inch tape on this quilt.
You'll be amazed at how many widths you can find. My favorite is the 2 inch. I can always eyeball adding an inch or so as I sew along the side of the tape on my quilt.
I lay 2, 2- inch strips down on this quilt to create 4 inches.
You can sew your grid design through out your whole quilt, or just portions of it.