Doesn't look like there's been a definite answer to this yet, so I'll give it a try. Don't have the game on this computer, though, so I can't post screens. Sorry :-(
The way you make your (techie) followers make things is to put the items in their inventory and talk to them. For example, Magnus can make spike traps (and more as he levels up, but at least those). You'd put the large spring and the railroad spike into his inventory, talk to him, select "What kind of things can you make for me, Magnus?" and it pops up the schematics screen. Then you just select the spike trap and hit go. It's the same with Jayna and herbalism and whatever other techie chars you'd get.
As others have mentioned, if a character doesn't learn something from their leveling path, you can't teach it to them, though...
I hope that helps!
Post edited May 09, 2011 by jurijchrul