For "Pop-Ups" my focus was on paper engineering. (The "E" in STEAM/STEM education). We started with pop-up cards to loosen up, and ended those with a flower-based pop-up that we re-interpreted as a splash, flame ball, explosion, or flower based on THIS youtube tutorial.
I had some patterned paper I got from Nasco I made available to them for carpets and wall art. Construction paper scraps were also helpful.
I pre-beveled paper to make folding easier by laying 4 pages on the edge of a paper cutter board, using the grid, pressing along the cutting edge with the blade up and away. This gave a light crease that was easy for students to re-crease quickly. We could have measured, or used the width of a ruler to draw lines, but with time as a factor, I did this and it worked really well.
If I did this again, I'd find a clearer plastic, ours was slightly foggy, but it could have been smudges from hands... We also did our images on the plastic with sharpie and painted to backs with white opaque paint to make it stand out. It may have been easier to cut out elements and put them on the plastic with double sided tape (which I didn't have) but these came out well. There may be many ways to do this project, feel free to share your tips in the comments.
My unique take on this is that we don't paint the plaster, we cover it with colored tissue paper. I used acrylic medium for glue, but you can just use watered down white glue, Mod Podge, or any water-based glue. Coat the area with glue, add small ripped pieces of tissue, and more glue on top. Overlap paper to cover the whole object. At the end you can add some details with paint markers or acrylics. Tissue can be cut to add patterns on top as well. The best reason for using tissue is that it does not flake off the surface like paints do, and it seals the surface with a bold color that looked great on these sculptures. It is also a technique from Japan, used for centuries, though usually over clay.
We used accenting colors of yarn to sew, awls to help make pilot holes, foam core scraps to protect tables, and fat needles that are easy to thread. Everyone struggled in the beginning, but quickly learned and helped each other. All, ages 7 through 16 did well. We added some elements with hot glue along with googly eyes I had leftover from last year. Be sure to not use hot glue directly on the eyes because it's too hot. I squirt a bit on paper, mix it with a popsicle stick and then apply it while still sticky. That seemed to work well.
Using rubbing alcohol on paper we ran it through a press to transfer the images. We found that you must wait for all the alcohol to be absorbed by the paper before pressing; puddles left unpleasant runs. We did a second "ghost" print for fun too by spritzing the actual acrylic plate.
This could be done with transparencies if plexiglass is not available.
I am sure this would work well with water-based markers. We may try those next week.
We did this project earlier in the summer without the use of a press, and though it worked, these images were much more saturated and crisp.