steam button → power → switch to desktop
start discover from task bar or all apps
search for “heroic games launcher” (x for virtual keyboard, left touchpad mouse movement)
install
start heroic games launcher
login → epic games

open steam desktop client
toolbar → games → add non-steam game to my library
tick box of heroic games launcher
click add selected programs
close steam
go back to gaming mode

https://www.xda-developers.com/how-to-install-play-games-epic-games-store-steam-deck/

To access sd card for heroic games launcher, use this command with the path to the sdcard:

flatpak override --user --filesystem=/run/media/mmcblk0p1/ com.heroicgameslauncher.hg

(make sure it matches your SD Card address), or use Flatseal

https://m.youtube.com/watch?v=HrTg0mDm2nA