or at least from McLeod Ganj:
1. Get it packed. This is important, and the most fun part of the process. Shops around town advertise parcel packing; go to one and a guy will cut and stitch a cloth wrapper for your box. His precision will be remarkable. Then he will seal it a few times with a wax seal. Write your destination (and your return address, i.e. your guesthouse here, even though this doesn't make any sense because if it gets returned you will not be here) on the cloth in a magic marker.
2. Get a customs form. I got one from the printing shop next to the post office. I have no idea where you get it if you're not in McLeod Ganj where there's a printing shop next to the post office.
3. Get two copies of your passport and visa. The printing shop is good for this too.
4. Wait in line and pay about 1 rupee per gram. (this may have been a coincidence, but it was about spot on in my case. really, of course, you just pay what they tell you. there are alternatives like fedex and dhl, but they're a little more expensive.)
5. Hope that it's before 1pm. It closes at 1pm. You can imagine there is not much leeway.