After installing it, log in with your Nexus account, and select Skyrim: SE as the game you're modding.
I highly recommend Vortex since it has a great UI and built-in tools to easily manage mod file conflicts.
These tools automatically handle most mods' installation, so you don't have to dig around in-game files - simply drag-and-drop compressed mod files, and the mod manager will do the rest. The first thing you need is a Nexus Mods account, the website where most mods are posted.
