Custom Build
The custom Forge builds that I make are distributed through my Magic the Gathering Nextcloud share. Under the forge directory, you should find both custom-builds and custom-builds-full.
The easiest way to install a custom build is to use the file in custom-builds-full.
First, remove everything from your forge installation except your forge.profile.properties (if you have one). It's also a good idea to make a backup of this file in a different location, just in case it does get deleted.
Second, open the newest build (filename like: forge-gui-desktop-X.X.XX-SNAPSHOT.tar.bz2) in the custom-build-full and extract its contents into your forge directory.