>>3287311I always recommend getting a set of good calipers. Dial calipers are great since their isn't a battery. Metal calipers can scratch things like lenses. Plastic calipers are perfect for working around cameras and lenses. Make sure it includes a depth gauge. Once you have this important tool, you'll be able to do precision measuring.
Making a lens is really easy. What is difficult is making the focusing mechanism and adjustable aperture. A lenses' focal length is measured from the inside middle of the piece of glass/plastic, not from the the front or the back; that's from the middle of the lens to the film/sensor. For making mounts, you can go the cheap route and use body caps. Just cut out their centers so the light can get through from the lens. Wider is better, but there's a limit. More expensive, but stronger are reverse rings for using a normal lens as a macro lens. You can just use one of those as your mount. Everything between the mount and the front lens can be pretty much anything you want. You can machine everything from aluminum blocks, 3D print it from plastic, or get out a roll of packing tape and cardstock from old cereal boxes and Pringles cans. So long as it is light proof you are good to go. FYI, 3D printed stuff needs to be painted well as it tend to leak light readily. If all else fails, just make a pinhole.
You are really only limited by your own skill and knowledge. Try with single element lenses with a fixed aperture first then work up to lenses with multiple elements and adjustable apertures.
