How do you make a map on counter strike source

You can make a CS:S map by using Souce SDK's Hammer World Editor

if you have a steam account and CS:S you already have SDK, to load it to your PC just go to your steam library, then to the tools section and under tools should be Source SDK once you download it (it shouldn't take as long a CS:S to download) go the the Hammer Editor icon and then design your map.
If you need mapping help you can go to Interlopers.net or developer.valvesoftware.com, if you own HL or team fortress you can also make maps for those as well.

Hope this helps :)