Fixed Launcher isn't absolutely necessary to run mods, but its very helpful because it will remember your load order between game sessions and alert you if a mod is missing its requirements. The native launcher works but you have to reorder the mods each time you play.
Other than that, mods are pretty simple. You extract the mod into the Modules folder and activate in the launcher and you're good to go. Make sure you always read the mod description carefully. Some mods will require other mods to run properly. Load order can be important for some mods. Required mods need to load before the ones that require them. Certain mods need to be loaded before the native mods and so on, so make sure to read everything on the mod page. And be careful with mods that look like they're affecting the same game systems as there can be conflicts if multiple mods are trying to change the same files.
Most mods can be safely uninstalled mid-game if you don't like how they work, but some shouldn't be. I think mods that add new troops or new items can cause issues if you uninstall them mid-game, so be careful of that.
And make sure the mod is compatible with the game version you're on. Each patch tends to break earlier mods, so you need to make sure the mod you're using is up-to-date. I recommend browsing Nexus by "Last Updated" and only using mods that have been updated since the patch you're on was released. Some older mods still work fine so if you want to try one, make sure to check the comments and see if other people have been able to run it on the latest patches.
Once you've got a nice modlist that you like and want to start a long playthrough, you should make sure to set Steam to Offline mode, so that when TW releases a new patch, it won't break all of your mods and you can finish your run in peace.