When my son was a baby I took a class in photography and one thing I remember the teacher saying is.... get your photo set up, then take one big step closer. Most books on composition mention cropping. I use my view finder for this. I have a couple. I'll included photos. The one you make is basically two "L" shapes pieces that you can adjust so easily for size.
The idea is to close in on your subject, simplify, and think about big shapes. The value viewer works for this, too.
There are a lot of good examples of compositions that are well cropped on pinterest and Daily Paintworks. Carol Marine's ArtByte has some great examples. I'll include one of of my own.
This helps frame and crop the subject you want to paint in a landscape or a still life set-up.
These can be adjusted to any dimension, I like them both, but the second one you can make yourself.
This is a 6 x 6 of mine where I did some serious cropping as a part of planning this composition.