how do I set up a "Cycle area??"

[fjs08]

I read with interest the concept of creating custom ring tones. Since my files on iTunes appear compatible with Garage Band, I was able to copy a song file to Garage Band. Now, the directions say:

>>Set up a cycle region that covers the area you wish to use in the GarageBand song. Note: This area must be 40 seconds or less in length.<<

I've never used Garage Band before. How do I go about setting up this cycle region.

Thanks.

Frank