There are several good step-by-step tutorials online, here is a good one.
http://www.dd-wrt.com/wiki/index.php...eless_Repeater
Note:
If you have it working with your normal internet connection. SAVE that configuration.
Reset to default configuration, then configure the repeater mode.
When that works the way you like. SAVE that configuration.
Then you can switch back and forth by just loading the different saved configurations.