heres a rough quick guide
select valheim as default in Thunderstore
then select game version ie steam or Xbo
i have steam so we may now differ
if you have installed a mod then on left you can go down to "edit config"
now my VERY limited experience says Bepindex( my slang for Bepinex)
needs to run Valheim plus.. so edit the Bepindex config , will have
BepInEx/config/valheim_plus.cfg and off you go