I followed a way explained in the BepInEx manual to install the plugins outside of the Valheim directory.
I created a /unity/BepInEx_Valheim/ directory in what should be \AppData on Windows (/.local/share/ on Linux), and put BepInEx and all its mods and libraries inside. Then there's only a path to change in a config file.

If you look under the requirement's sections of those mods they both show that they require the BepInExPack mod located at:

https://valheim.thunderstore.io/package/denikson/BepInExPack_Valheim/

The BepInEx mod ^ must be installed prior to any other mods. ( so that the BepInEx folders actually exsist for you to place mods into).


After that you will need to Install the JotunnLib mod from:

https://www.nexusmods.com/valheim/mods/507?tab=files

This set of libraries ^ is required for the Necromancy Conjuration Mod you were wanting to install.

Then you should be able to install the two mods you linked to.

(I hope this helps. I am stepping out for a few hours to get some stuff done before the weekend but will be back later to check up. Good Luck and Good hunting!)

(your install drive) -> Steam -> steamapps -> common -> Valheim -> Bepinex

Under there you'll find the folders config and plugins.

I think that NM, has valheim support with their vortex mod manager, it's easier to do modding with that