Download Vortex [then add your Skyrim SE to Vortex. Vortex is your mod manager I use this as i find it the fastest and easiest way to install most mods automatically without having to manually drag and drop tons of files. Firstly you will need an account at NexusMods [set that up go premium or don't you can get all the mods for free but downloads will be slower for non-premium accounts.
