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

I've also collected most of these into an editing starter pack for Windows, which you can download here. See below for instructions on setting yourself up!

Setting up

Here's a video describing how to set yourself up for creating Doom levels with the contents of the starter pack.

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). This video shows 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. However, if you have a WAD that doesn't fit through the uploader, you can get in touch with David through the Doomworld thread or on Discord. This video shows how to use the uploader.