This guide is an outline of my experience with running FTB Revelation on the Steam Deck. I believe many others may want to play a modded version of Minecraft and I have discovered this modpack's requirements fits within the performance of the Steam Deck. I will try my best to update this guide as I find more issues and fixes while I continue playing.
The first step is to download PolyMC from Discover. You must exit to desktop mode on the Steam Deck, open Discover, and install "PolyMC". Once that is done, open the desktop Steam client. From there you will add PolyMC as a "Non-Steam Game". If you want the art to show up in the Steam Client, I recommend using SGDBoop.