Most will include instructions on how to install their particular mods so if you run into any problems, consult their instructions to see if anything was missed. You can check your Right to Rule rating in your Character Report.
Some mods may require additional steps if they include texture packs or other files that change the core gameplay. An important new concept in Mount & Blade: Warband is Right to Rule. If you have not modded your game in the past, it will be set to native by default. I've been a vassal of an empire for quite some time and i have 45 right to rule and i've also become marshal once before and almost became marshal a 2nd time but was outvoted by one of the top vassals.
From this point the steps are as follows: I will assume you know how to find the mod (or follow one of the links below to the mod location) and download it. For those of you who may be new to modifying games, the directions are pretty simple. Installing mods for Warbands is pretty easy for anyone who has modded a game before.