Which map to use is of course of your choise depending on the area of your interest. Use the map index post for guidance.
Find the best map, if it is in the form of one single .img file just download it to your computer. If it is a Mapsource installation, install it in Mapsource.
Since the 60CSx only can handle one map file the file needs to be renamed to gmapsupp.img and copied to the folder named garmin in the sd card so the the gps can find it. If it is a Mapsource map just transfer it to the gps from Mapsource.
In the case of locked maps unlocking is neccessary.
Information on how to do theese steps can be found in the forum, use the Sticky posts to find out where !