First of all, of course you'll need to have a computer with Assetto Corsa installed.
Second of all, you'll need a mod you'd like to download. The Honda NSX is an excellent example. Click on the blue "download now" button and save the .zip file on your computer. If you like to be organised like me, save it to C:\mods\assetto corsa\cars or something like that.
Download link for the Honda NSX, Honda NX-R and two variations of each (all four come with the one .zip file)
http://www.racedepartment.com/downloads/honda-nsx.4768/Open your Assetto Corsa folder from My Computer -> Program Files -> Steam\Steamapps\commons\assettocorsa\content\cars.
Unzip the AC_HondaNSX_beta2.zip file to the folder you wish. Open the AC_HondaNSX_beta2 folder and copy&paste the four folders containing the two cars and two different tuning options into the previously opened My Programs...content\cars folder.
Open Assetto Corsa and enjoy the great car! If you can't see it right after Ferrari and before KTM, you've done something wrong.
For tracks, unzip the file to ...content\tracks and for skins, unzip the file to ...content\cars\THECAR\skins where "THECAR" means the car the skin is for. For example ...content\cars\honda_nsx\skins.
Bonus tip: if the mod is working, the .zip file is useless as is the file you extracted from the .zip file, because it has now been copied into the correct folder. Delete these to save space on your computer! Alternatively, you could just cut&past, or in some cases, just unzip directly into the assetto corsa subfolder.