How do I convert an existing electrical panel to a subpanel?
Main breaker

There's no problem with having a main breaker in a subpanel. It's required if your panel is in a separate building and has more than six breakers, but there's nothing prohibiting it in any other case. 

Wire size

You need to ensure the wire between the panels is sufficient size. See What wire gauge do I need for a 100 Amp subpanel at the end of a 60' wire run?

Neutral-ground bond

You need to ensure the only bond between ground and neutral is in the main panel. The subpanel must have separate ground and neutral bus bars. Since your panel is used a main panel now, it will currently have a bond between ground and neutral, so that'll have to be removed. 


It's usually a metal bar or connector that screws between the two bus bars, or between the cabinet and neutral bar. 
If you currently have neutrals and grounds mixed on both bus bars, you'll have to separate them out
If you have only a single bus bar (with grounds and neutrals mixed) you'll have to add a neutral bar (which is not bonded to the cabinet). Check with manufacturer first to see if they have a documented way of doing this. If not, I'd suggest you check with your AHJ (inspector) first to be sure whatever you do will be acceptable.