When you have the instance basically set, go back into the AWS control panel, right click the instance and choose Create Image. You can start up any number of these for the replica set, but you need to change the /etc/hostname and /etc/hosts file to reflect the individual IP address and hostname of the bot (db1, db2, db3, etc.)
Before you set up MongoDB on EC2 make sure you understand the various aspects of running MongoDB in the Amazon cloud:
This blog is called myNoSQL and it is written by me, Alex Popescu, a software architect with a passion for open source and communities.
It records my readings, learnings, and opinions on NoSQL databases, polyglot persistence, and distributed systems -- subjects that I'm passionate about.
The opinions expressed here are my own, and no other party necessarily agrees with them.
If you feel I'm biased, I probably am.