Most 3D printers come fully assembled and ready to print, but some are sold as kits that you assemble yourself. The prebuilt models are easier start with, but the kits offer better insight into how these printers work, and allow you to customize and tweak them yourself when they are up and running. There are a lot of parts, but most kits require just basic mechanical knowledge: If you've put together an Ikea bookshelf, you can build a 3D-printer kit.