Most mods will have a Read_Me file that will tell you how to install the mods. Just Cause 2 is actually really easy to mod (easier than most games I've seen), as it doesn't require you to actually replace files. Majority of the mods out there are as simple as copying the included files into a "dropzone" folder in the Just Cause 2 folder (You have to make it yourself, just a normal folder named "dropzone" in the same folder where your JustCause2.exe is). But a *few* are different and you'll need to read the instructions for each mod to make sure.
its pretty simple stuff, go to a jc2 site, namely www.justcause2mods.com find whatever takes your fancy, download it, unzip it and unless you have the yyaarrrr'd version put it/them into a directoy like this:
C:\Program Files (x86)\Steam\SteamApps\common\just cause 2\dropzone
the dropzone file wont be there originaly, so just make it yourself, run the game and voila, mods are active
I put the laughter in manslaughter PSN- tomma101 Australia, love it or leave it