Making a pk3 - Its kind of an easy thing to do, but there are a few things
that really need to be done right.

Make a zip file called <mapname>.zip with the following directory structure;

/maps/<mapname>.bsp
your map file

/maps/<mapname>.aas
your map's bot file

/levelshots/<mapname>.jpg
your map display shot

/textures/<mapname>/<mapname>_<texturename>.jpg
a custom texture

/textures/<mapname>/<mapname>_<texturename>.tga
another custom texture

/scripts/<mapname>.arena
makes your level turn up in the menu

/scripts/<mapname>.shader
a custom shader for your map

/<mapname>.txt
some info about your map

Once you have made the .zip file you need to rename the <mapname>.zip to
<mapname>.pk3

If you use Microsoft Windows, you may need to enable file extension. You
can do it as a folder option from the Windows Explorer menu. Then place
the .pk3 file into a zip file with a copy of the readme.txt file.
Placing it in a .zip makes for easy download as some servers/browsers
mis-read .pk3 as text (because of MIME settings).

Thats about all there is to it.