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)

You will need

Setting up

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). 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. This video shows how to use the uploader.