How to make white grids?

Use either photoshop and don't be a retard or indesign an don't be a retard. That's as hard as it will get.

Adjusting measurements is something that design programs still don't do well, despite the fact that it's been in (2d) CAD programs for literally always.
Anyway in photoshop I would use guides, e.g. put one at 5%, 47.5%, 52.5% and 95% and you'll have evenly spaced horizontal borders.

File: 123.jpg (240KB, 612x792px) Image search: [Google]

240KB, 612x792px

>>2951331
never done diptych before. fuond presets in lightroom and did this. idk about ps

>>2951490
>diptych
what preset?

>>2951550
The one that says custom (2) 5x7 aligned