how do i install nexus mods on skyrim se without breaking it

Once you have more than a handful, run LOOT to sort the load order, and make sure SKSE matches your exact game version. Skyrim updates have broken SKSE plugins overnight more than once, so set it to only update when you launch it and always start through the mod manager.

Use a mod manager, never drag files into the Data folder by hand. Mod Organizer 2 is the safest because it leaves the game folder untouched, Vortex is easier if you just want it to work. The thing that breaks most setups is versions. A lot of mods need SKSE, and SKSE plugins are built for a specific game version, so when Steam updates Skyrim they stop loading. Check which version you're on, grab the SKSE build for that version, and read each mod page for which version it supports. Set Skyrim to only update when you launch it so it doesn't surprise you. Add a few mods at a time, run LOOT to sort the load order, and test before adding more. When something breaks you'll know what did it.