1. Bring ID, and 210 peso cash or a credit card
2. Go to Telcel store "Centro de Atencion a clientes Telcel" on Eje Central Lázaro Cárdenas. From the Hilton Reforma, right across the busy street, just before the "Empire State building".
   
3. At reception, say that you want to buy a chip Telcel Amigo
4. You will get a ticket with a desk number to report to. Probably upstairs.
5. Go to that counter
6. Ask for Telcel Amigo 7 dias 200 mega
7. Costs 150 for sim, 59 for the data
8. Computer stuff is done, forms are filled out
9. You are taken to the payment area. Pay there. 150 for the sim
10. Back to the clerk's desk
11. You'll get the SIM card
12. Put it in the phone and wait until the network is discovered. Telcel 3G for me
13. Hand the phone to the clerk
14. They will call a number. Presumable to activate
15. The you have to pay a second sum for the data. 59 pesos. You'll get an activation code. The clerk entered it for me and showed me the text message that confirmed the 200MB of data.
16. All done. Phew. Enjoy your data.

This took me about 45 minutes, including the walk to and from the Hilton. Feel free to publish this somewhere on Wikimania wiki, or tell your friends...