The installation steps are pretty much the same because the Steam Deck runs pretty much a modified Arch Linux installation.
Thanks EmeraldWither for this section.
Switch your Steam Deck to Desktop Mode.
Install the protontricks Flatpak. (This can be accomplished via the Discover app)
Follow the instructions here. Choose the protontricks options.
You'll probably want to set up an automatic script that launches BakesMod alongside Rocket League. If that's the case, follow the instructions here.
Whenever you want to use the BakkesMod window, hold down the Steam button and use the right trackpad and triggers to use the mouse.
Opening the BakkesMod window is done with the F2 key (and closing with Esc). I suggest you bind those keys to some buttons on your Steam Deck.