Step 1: Friendly Plastic Introduction
I've used it here to make a duplicate for a hard-to-find plastic wrench from a child's construction toy that is no longer easily available.
Step 2: Visualize the Shape & Mass of the Tool You Want To Build
This toy has a variety of colored connector tubes and white tubes of various length.
This blue connector 'locks' into the white tube. The gray knobs in the blue connector stick out into holes in the white tube. The gray wrench has two little nubs that stick into the holes and push the gray knobs in so that the white tube can be removed. Without one of the gray wrenches the lock fit is very snug and hard to unlock or manipulate because the gray knobs are separated 180 degrees on opposite sides of the tubes.
My goal is to make a work-a-like for the gray wrench. The cosmetics are not important.
In order to measure out the Friendly plastic pellets, you need to a have a rough idea of the mass of what you're building. No problem if you overestimate a bit as it is completely reusable.