Hi, Lucia.
After you have packed custom content and built your cubemaps, you can perform a
repack to compress your map before you distribute it.
The easiest way to repack a map is to open the console in TF2 and type the following:
bsp_repack maps/mymap.bsp maps/mymap.bsp
Replace
mymap.bsp with the name of your map. This command string will compress your map and keep the same filename. The results will be printed in the console, and it will show the old file size and the new size. It will take longer to compress a large file. If you want to have a different filename for the compressed version, change it in the second parameter. make sure you make a backup of your map before you compress it.
For more detailed information about repacking, see
this thread.