To add this item to your NWN2 character you can use the in-game console, the Silverwand Item Summoner, or purchase from the Silverwand Magical Emporium.

To add an item with the console giveitem command or the Silverwand Item Summoner you will need to know the item's tag, which is the part of the file name preceding ".UTI". After subscribing navigate to the appropriate sub folder in C:\Program Files (x86)\Steam\steamapps\workshop\content\2738630
If you look at the contents you can see which files are UTI files. If there is documentation you should read it,

Using the console giveitem command will disable achievements, so to reenable achievements save the game and then edit the file globals.xml in the saved game folder, changing the 1 to 0 (zero) in these lines (credit to Aldroen):
<Booleans>
<Boolean>
<Name>ShowCheatsWarning</Name>
<Value>1</Value>
</Boolean>
<Boolean>
<Name>Cheater</Name>
<Value>1</Value>
</Boolean>
</Booleans>

There is no need to install. When you subscribe it is downloaded in a way that makes it available immediately. There isn't any way for me to fix what the ring can do. If it doesn't work for a race then that is just a limitation. I typically play male half-elf, elf, or half-drow characters, so I did not notice it was not working for humans.

Okay, thank you. I'm going to try these two first. I'm really looking forward to your ring succeeding. I downloaded it, but I'm not sure where I should install it. Could you guide me?

There is an alternate way of changing human heads to skulls by another modder available here: https://neverwintervault.org/project/nwn2/model/skull-heads
I do not use it myself and I cannot vouch for its compatibility with NWN2 EE.

And there is another method that requires installing a model here:
https://neverwintervault.org/project/nwn2/hakpak/original-hakpak/skull-head

I've done some testing with the SoZ cohorts and have posted the results in the description above. I do not know if specific head types for the different races matter. Too many combinations to test.

Hello, I really like undead creatures. I used your MOD, and my created character is human. I successfully added the ring, but when I used it on myself, instead of turning into a skeleton, the character model disappeared completely. What could be the reason for this?

Note, this may not work for all race types.