If this is a Whirlpool manufactured dryer (which inlcudes Kenmore, Roper, Estate, Kitchenaid, and Frigidaire), just follow these steps:
a. Loop the drive belt around the dryer drum with the groove side facing the drum. (Try to line the belt up with the wear marks already on the drum from the previous belt).
b. With the tensioner in your left hand, insert the clip end of the idler pulley into the grooves on the dryer bottom casing with the idler wheel pointing to your left. (The grooves are directly in front of the drive motor.)
c. While holding the tensioner in place, insert the belt
UNDER the wheel with your right hand and pull it up and over the drive motor pulley.
If this does not explain it well enough, you can also verify by going to repairclinic.com. Click on the Repair Help link and look for the "Dryer Belt Diagrams" link under the Special Features section on the next page. Look for Whirlpool
and locate diagram H.
NOTE: It is normal for the idler pulley to come loose when there is no belt tension.
If your dryer is not manufactured by Whirlpool, or if you simply aren't sure, please post back with your model number so I can give you more accurate information. I hope you find this helpful.