You will need BSPSource, the map and your brain.
1. Download BSPSource - http://ata4.info/bspsrc/downloads.html
2. Unrar it to its own folder.
3. Run bspsrc.bat.
BSPSource Will Now Open
4. Press add, find the map you wanna decompile.
4b.(Optional) If you want all the textures(.VMT and .VTF files) too use on your own maps,
go to the OTHER tab and tick on: Extract Embedded files
5. Press Decompile and choose where too save the decompiled files.
If you have done it correct you will now see this. (This is with step 4b, Thats why there is a Map folder also)
Disclaimer: This will not always work as you can prevent people from using these programs with a obfuscator