This is my first go at making something in tinkercad, you can see its on the 4th version, but i had a lot of fun making this.
all of the pieces should fit together fairly snug, sorry i don't have a diagram. Think of it like a puzzle :P I will upload a picture of what it is supposed to look like soon.
This does need one or two rubber bands, they will loop onto the two hoops on the short pieces and hook onto the pegs on the center piece. you can wrap it around multiple times to get a stronger launch.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/272r5keTstM