Nexus is fiddly due to how their mod manager works. Lots of elevations etc etc to play with per mod.
Thunderstore is a much simpler process. Search for the name of the mod you want. Click it, click install, done. And if you dont want it anymore just hit the disable button.
I havent had a problem with mods from either source, mind you. Its just that for me at least Thunderstore is easier to use.