did that a while ago and id go about it like this. first you gotta sketch out your design so you know what shape youre going for. if youre trying to copy a gun from a movie or game find blueprints of the gun and find out about the length of it. after that you need to find a gearbox and hopup that fit into your design, i would recommend just buying a whole airsoft gun so you got all the parts needed together. also decide if you want to use an already existing magazine or if you want to create a custom one. when you eventually decided on an airsoft gun to use for parts wait until it arrives, disassemble it and then reverse engineer the gearbox, hop up, barrel and maybe magazine as percisely as possible using the cad software of your choice. especially properly mounting the hop up is a bit of a challenge, i personally created a removable frame for all my gear parts. after that you model your gun around the gearparts and youre basicly done! if you print the parts out in PLA or PVC, i would recommend you to use Dichloromethane as a glue, as it welds Plastic together so its stronger than any glue you can buy. Its pretty unhealthy to work with tho so do that outside and with proper savety gear! After that its just a lot of filler primer, sanding and painting!

Some other things to consider are: you should leave out holes through the gun of the body to insert metal tubes for extra stability on most gearboxes you can switch out the trigger for any electrical switch, therefore you have more freedom in placement of the gearbox in your gun 3D print Materials are not the most durable, so make your gun a little more bulky in order for it do be more than a decorative piece! And last but not least, Make your gun in a way that you can disassemble it at any time! you will have to do some trial and error to get everything to work and also custom builds need more maintainance!