Here is how the Timer website explains the process :
Timer is a Twitter bot that helps you set an alarm for things you need to remember. Want to give it a try? It’s easy:
If you’re not already a Twitter user, register for a free account.
Add timer as your friend.
Send a direct message to timer. If you’re sending a direct message via SMS/Twitterific, use the format below (if you’re using Twitter’s direct message interface, you can leave off the d timer bit. More info on direct messages can be found here.):
d timer [# of minutes until reminder] [your message]
Example: d timer 60 check parking meter (will remind you to drop a quarter in 60 minutes)
Note: 5 minutes is the minimum amount of time that you can set. In addition, if you forget to put a time, that’s what you’ll get by default.
Timer will then tweet you back after the specified number of minutes has passed. Please note, though, that it is not an accurate timer; many factors, including email, SMS, and Twitter delays may slow delivery. That said, it’s usually not off by much more than a minute. Keep this in mind when you’re using timer to cook macaroni (please don’t hold us responsible for soggy noodles).