After installing it, log in, and select New Vegas as the game you're modding.

I highly recommend Vortex since it has a great UI and is very easy to use. 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. VPN Deals: Lifetime license for $16, monthly plans at $1 & more