1. Insert all red items
2. Insert green item when all fleas are in the bottom-right "rectangle" of the level
3. Insert cyan items after all fleas get stuck behind the one-way ramp. Try not to trap any of them.
Repeat this process for every other section. (Note: Feel free to put the "one-way" ramps a bit farther from the boundary walls of the level - it will make it easier not to trap any fleas while you're doing the level.