The situation: I need a roundtrip from SFO to SCL (Santiago, Chile). I want to buy a coach ticket and upgrade with miles. I'm CO Gold Elite.

The choices: The best option seems to be a Delta flight from SFO -> ATL -> SCL. It's code-shared with Northwest and Continental and NW's prices seem to be the lowest. CO's price is 2x NW's price on the same exact flights. Go figure.

Stupid/naive questions:

- How do I actually book a flight with NW and get CO miles for it? Is there some way to do it online from nwa.com? Third-party like Orbitz?

- NW's web site says "select coach fares" cost 20K miles to upgrade one way while others are 10K miles. Which gets what?

- Any other advice/gotchas about how to properly make this reservation?