There are a ton of guides for editing Doom out on Youtube - here are a few resources to get you started!
Eevee's guide (focuses on mapping with Slade3)
Linguica's guide (older but still good)
You will need
- Either a copy of DOOM2.WAD or FREEDOOM2.WAD from Freedoom
- A source port for running Doom levels (e.g. PrBoom, GZDoom)
- A DOOM level editor (e.g. Ultimate Doom Builder, Slade3)
All of these are provided together in the RAMP Starter Pack.
Here's a video describing how to set yourself up for creating Doom levels.
Making a map
Doom maps can come in several flavours. You might want to stick to the map features possible using the original game, or experiment with the wide array of options added since then in the Universal Doom Map Format (UDMF). These videos show some basics of building a map with both.
Submitting a WAD
The easiest way to submit a WAD to RAMP is through the uploader on this site - you can use the PIN given to you after the initial upload to submit further edits to your WAD once it's been submitted. This video shows how to use the uploader.