thanks all, got it working

Get vortex to manage, nexus to get your mods. They are integrated, easy to use.

Learning vortex itself is a whole thing if you're not familiar with it though.

Manually, your starting point is to install https://valheim.thunderstore.io/package/denikson/BepInExPack_Valheim as described in the readme, copy mod .dll files into valheim/bepinex/plugins and edit mod settings in valheim/bepinex/config

The thunderstore mod manager is great, supports few games but very easy to use.
I don't know why they went the bloated and obstuse direction they did with vortex, NMM used to be great too.