Tape measure: https://amzn.to/46ybrnU
4 foot level: https://amzn.to/3QZnGUJ
3M Claw picture hanging (best option!): https://amzn.to/46DeXNI
BEST method of erasing pencil marks!! Magic Eraser: https://amzn.to/3RhnZf2
Pencils! Never can have enough! : https://amzn.to/3R0MbRh

All you need is height & length of your picture along with the "gap" between frames and a gallery wall is easy to setup. Add Length of picture plus Gap...take that number and start hanging!