Modern computers and printers give us a great amount of scope for designing 3D models and decals. Anything we can imagine can probably be made with the right computer package.
Of course this is all for nothing if we cannot come up with an artistic design. Having the tools and facilities is one thing; having the inspired idea is more difficult.
If we are building a reproduction of a real vehicle, military planes or model trains, then decals are based on actual signs and insignia. Getting the size right is important, as are the colours. Otherwise we can use any printer and waterslide decal paper to make the appropriate designs.
Most Scale models use scale model decal paper. Thin decal paper is a better option if the surface of the model has surface details or if the decal goes over an edge. Thin decals will suffer far fewer air gap problems as they can accommodate the fine 3D details of a hard surface.
Blue backing decal paper is used by some scale model hobbyists as it allows them to see the white colouring of some decal designs.
Many individuals like to decorate laptops, phones or musical instruments with their own designs.
Using an image from a cartoon, film or any pop culture source is one option. As these are not being used commercial purposes there should be no copyright issues.
Metallic decals are a good option for many computers or phones. But many just uses images on inkjet decal paper.