Yea, we can't exactly be much help without an area of interest.
Outside them? Tools are a big necessity (files, a dremel, pliers, a vice...at least)
Epoxy putty works wonders - fills any gap, space or piece you need - can be filed, drilled and sanded when it dries. Oatey sticks in the plumbing dept works well, along with hobby epoxy putty like sculpty online - try to stay away from Loctite sticks, unless you need something you can sculpt in less than 2 min.
Super glue is not that super. It can be brittle and epoxy glue (2 part) will bond much better. JB weld is kinda awesome too.
Time, patience, and intuition/creativity play a big role.
when looking for free parts, check re-use places, recycling bins, old junk you have lying around (even think - you can cut it up, take it apart, etc. phone charger? you've got rubber, wires, transistors, etc) even check dumpsters, or estate sales.
plan first.
things look much better after painting. cobble something together, now spray it with black paint. Admit it, you're a little happy with yourself
plumbing tubes are measured from the inside.
learn to solder if you have a few hours, it's a very useful skill even outside prop building.
I..I'm just rambling, I'll shutup now.