Starting Guide

Choosing a map editor
In order to make your first map, you need to download and install a map editor for Doom. This tutorial assumes that you already have either Doom 1 or 2 installed (Doom 1 shareware will not work; it does not allow you to load custom WADs). When looking for a map editor to use, you have these options (plus more not listed here).

Eureka Doom Editor
If you don't really care, and just want something that works universally, I recommend Eureka Doom Editor (the download is on SourceForge, linked in the the repository's README). It's a map editor that's been around for quite a long time (over 10 years!) but still receives occasional updates as of 2023. It has all the basic functionalities that you will need for targeting PrBoom+, Chocolate Doom, Crispy Doom, etc. It's the one I personally use, but it isn't a very popular choice because there are other options that have more community support, such as the next option here.

UltimateDoomBuilder (UDB)
I've never used UDB, but it's really popular among Doom modders. You can find UltimateDoomBuilder at its GitHub repository.

Learning the basics
Ultimately, the basics of creating Doom maps is quite straightforward and stays the same between source ports. Every map can be broken down into four elements: Sectors, Linedefs, Vertices and Things.