Just downloading and installing mods with a manager is going to give you crash to desktop most of the time. Modding takes a lot of practice and there's a lot of techniques to master. Honestly the best thing you can do is a lot of research. I got everything running real smooth aside from neck seams which I need to go back and fix.
I actually just bought and modded New Vegas for the first time yesterday. I use mod organizer and highly recommended it.
