Step 1: Download PRISM LAUNCHER.
Step 2: Log in to Prism Launcher with your Microsoft Account (Mojang accounts are no longer supported).
Step 3: Select Add Instance, select Import on the left. Paste one of the URLs below. Use the 6GB version if you plan on running Distant Horizons on top of all graphics packs:
http://friends.nullcyber.space/static_pages/friends/NullCraft.zip
http://friends.nullcyber.space/static_pages/friends/NullCraft_6GB.zip
Step 4: Launch the Instance.
When asked for a fingerprint/certificate, paste the following key:
KEY HERE
Step 5: When downloading the modpack, make sure "Keep Existing Files" on the bottom left is CHECKED. Click Customize and check which features you want on your client. Download the modpack and restart.
Your instance is ready to play on the server.