Most of the route should be clear from the WR run, in this guide I'll clarify some of the details that may not be obvious at first sight. WR as of writing this guide can be found at https://www.speedrun.com/usefuldead/run/z51jwq5m

Critter count: The game gives you enough critters for the intended solution plus 10 spares, our route will be more efficient on some levels, while purposefully wasteful on others. In general you'll end with more then 10 critters remaining, so there is no reason to reset a level due to excess critter loss.

Self-kick: Normally, you need to kill yourself and then have the new critter move to the corps to kick it. By kicking just before dying the hitbox of the kick will be active on the now corpse thereby kicking it. This allows us to skip backtracking to the corpse to kick it with a new critter thus saving us some time on some stages.

Last level glitch: For some reason, being squashed while facing the the moving block and holding left does not kill us, instead we are pushed into the wall allowing us to jump and clip through the wall and pop up near the exit. This trick is theoratically doable by suiciding only one critter but I found two critters to be more consistent myself.