It would be super helpful for you to pencil yourself a grid if you don't already have one. (rootsandwingsfurniture.com)
• MINIMALISTS (1960s, 1970s) Agnes Martin (1912 - 2004) American Minimalist painter; hand - drawn pencil grids on gesso, acrylics / oils. (visual-arts-cork.com)
Echoing the compositional elements of vertical and horizontal lines that were characteristic of Martin's work, the Dear Agnes series is comprised of hundreds of pencil grid drawings in seemingly endless permutations. (remaimodern.org)