Wanting to make weapon viewmodel sprites but confused

The originals were made from photos, with spritework done over them.

Today the two most common methods are either old-school spriting or using 3d model rips / screenshots from other games.

If you want to learn how to sprite, the fastest way is to grab a few sprite sheets from a site like The Spriter's Resource and try doing recolors and splices to make new sprites out of these. Look for tutorials on those subjects - they apply to any kind of sprite.
Spriting is technical and doesn't require the same artistic skills as something like painting, but it helps if you have artistic inclinations.
After you get a feel for how to work with pixels you can then branch out to making sprites from scratch once you get good at those two methods.


Another method is to get screenshots of existing 3d gun models from a model viewer, or screenshots from a game, then pixelate them and color them in, or use filters to make them look pixelated. You can then use other spriting techniques to edit them further and make new weapons from those.