Print sled.stl and frame3.stl twice. Don't print trebuchetassembled.stl. Print 1 of everything else.

Assembly is pretty easy, if you need help download the assembled sketchup file. The supports will have to be glued into the bases. That's the only place where my print required glue.

The 45mm x 3mm bolt is the axle for the counterweight basket.

You'll also have to make a sling. I used duct tape and some cotton string (see image.)

When the sling is hanging from the arm, it the bottom of the sling should be at the main axle.

Use a drill to add a small hole (I used 1/16") to the top of the throw arm, go through the hole for attaching the sling and into the other side a bit. About 1/4". I'll upload my revisions as I make them, I'll add the hole in the stl file so no drilling will be required.

Cut a straight section of a paperclip and insert it into the hole, bend it back (away from the pieces printed in frame2.stl) to about a 45 degree angle. This pin controls when your sling with let the projectile loose, straighten it up a bit if it's releasing too late, bend it more if too soon.

Add counterweight to the basket, the ratio of counterweight to projectile mentioned on sites that know about such things is between 75:1 and 100:1. I use a bunch of brass keys.

It's all really "fiddly" for a bit, but once you get your weights, sling, and pin adjusted properly it works pretty well.

For more info on making adjustments see:
members.localnet.com/~lorilee35/tuning.html
thehurl.wikidot.com/tuning

Have fun and don't put your eye out.