How should I start making a doom game?

Total conversions are always cooler to see but they're a lot more work.

If you've never actually done any doom modding at all you should probably start by trying to make a level. One fully complete and playable, from start to finish, level with no bugs and make it actually fun. If you can do that, then try making a set of levels and that meet all of the above criteria and if you can do that, then try making a game.
 
I'd say start with asset design. Make a wall, floor, and ceiling texture and a weapon sprite. Then you can import your work into the level building tool. Nothing makes you feel like a real indie dev like seeing your own handmade graphics in action. Props for using the actual Doom engine instead of something less authentic like Unity.
 
Wstecz
Top Na dole