Build any blue level part in the factions work bench. Easiest way is when you unlock your secondary faction such as nomads or lunatics. Each item you want to build has different parts needed but usually involves scrap (got after every battle or raid or you can scrap an item) copper (found in raids) and 3 different parts - each you need two of.
Building with an engineer you may need to build parts first but usually another faction this will usually be parts found in any battle such as 2x radar, 2xavenger 57mm cannon, 2x car jack along with 50 copper and 450 scrap (this makes the at wasp in the lunatic bench)