For younger students, repeating overlapping shapes or lines create a haphazard pattern of shapes on a surface. The more random the patters, I find, the better. Grids and carefully measured patterns create rather robotic 8-bit images (Which is fine if that's what you want) but are more Pop Art connected than cubist.
Just as when you stare at clouds images start to form, the same happens with random lines and shapes. The human brain seeks order from chaos. This is called pareidolia.
SciShow on YouTube has an interesting video on the effect HERE.
For younger students, (K to grade 2) overlapping shapes works well, and reinforces skills like eye-hand coordination and shape vocabulary. The most simple version is to overlap 10 triangles, 10 circles, and 10 squares to fill a drawing page. Then students outline and color in objects they see. It is VERY helpful for the teacher to do this first in the front of the room and have students come up and point out what they see. It will also help clarify the goal of the project for others.
In middle school and high school, I have students criss-cross their page with about 50 lines with a ruler. Instead of finding images within the lines, I have students work from a specific image of their choice. It could be a cartoon character, face, landscape, or still-life. Students "force" the image into their random lines. This comes closer to cubism as they need to make visual choices to translate what they are seeing into an unusual format. I tell them it's like painting an image seen in a shattered mirror.
I have done this with animal paintings with younger students with some really fun results. You can see that some of their work below.
A third option is to do a drawing and then criss-cross it with lines and color each section differently. Below show some portraits done this way and we abstracted the faces by doing blind self portraits. (Not looking at your hands while drawing.) These too came out well. I encouraged those students to work with analogous colors so that the images would be more cohesive.