1. Create your map using the Portal 2 Puzzlemaker or open an existing one.
2. Go to "File" -> "Publish..." (Pic 2)
3. Wait, until the map is compiled.
4. As soon as you see the publishing-dialog, click cancel. (Pic 3)
5. Repeat set 1-4 untill all your maps are extracted.
6. Quit Portal 2 and lunch th P2-SDK to load the maps. (Pic 4)

(Picture 5 will show you how to get you mapID-name)

If you just run a map ahead, it will be saved as "preview.vmf" in your skd-maps folder, but as soon as you clich publish, P2 is compiling the map with it's unique ID. You might know, that you can't just rename maps because the lightmap will get lost. This way, the map would be allright and ready for upload.

Please note: Loading the map in Hammer could take some time because he needs to load all volxel-vmfs.