I need a diagram of how the drum belt is installed on a maytag centennial dryer. Mine came off and I don't know how to put back on.

It sounds like the spring which tensions the idler pulley has come off the bracket. You'll need to locate it and put it back on.

You have to take the back off of the dryer and see the motor pully and the idler pully. slip the belt around the drum, around the mototr pully and usind a flat blade putty knife, work the belt over the idler pully so that it puts tension on the belt. you may have to loosen the idler pully housing bolts, but try it first with working it over the idler as that was set for optimum tension.

Copy and paste this Factory Service manual for the Centennial dryer.
It has all your answers.

https://secured.whirlpool.com/Service/SrvTechAdm.nsf/2cd44500d572193285256a45004fd9d6/79aaf530992e93a0852572c3006587ed/$FILE/8178629.pdf

https://secured.whirlpool.com/Service/SrvTechAdm.nsf/2cd44500d572193285256a45004fd9d6/79aaf530992e93a0852572c3006587ed/$FILE/8178629.pdf
There is your service manual.

Go to repairclinic,com and click on the "Repair Help" link. On the following page, look for the "Dryer Belt Diagrams" link under the "Special Features" section. There is an extensive list of dryer belt diagram illustrations to choose from. Scroll down until you find Maytag. If you have any questions, please let me know. I hope this helps you.

Go to repairclinic.com and click on the "Repair Help" link at the top of the page. On the following page, look for the "Dryer Belt Diagrams" link under the "Special Features" section. Refer to Diagram E for a Maytag dryer. If you have questions, please let me know. I hope this helps you.

Based on the information provided, it sounds like a broken drum belt is at fault. Below is an illustration of the drum belt.
http://www.repairclinic.com/SSPartDetail.aspx?s=t-PYG2200+-%3d%3di529597&PartID=529597

Here is a parts/disassembly diagram:
http://www.repairclinic.com/Dryer-Gas-Maytag-Style-Appliance-Diagram

Here is a belt installation diagram:
Dryers have a drive belt that turns the clothes drum. If the belt breaks, the drum stops rotating. Then you need to replace the belt, and often the idler pulley, because the pulley tends to wear out at the same time.

Click here for a belt installation diagram.

All links courtesy of www.repairclinic.com

Ignore gas references if electric dryer.
Unplug dryer and shut off gas. Disconnect gas line. Tip dryer onto its front. Remove back panel. Unscrew and remove inside back manifold. Place belt around drum and fit into pulley wheel(s) on motor shaft. (There may be a spring loaded tension arm on one of the wheels. Install the belt on it so that it pulls the belt tight around the drum and pulleys.) Check friction gasket (that the drum rotates on) on manifold and replace it if worn thin. Install manifold. Install back panel. Reconnect power and gas.

if not able to remove rear panel,you will need to start,like most models,take top cover off and hinge back to wall then remove frt cover including door as a whole assy. lift up drum put new belt on drum ( and position in same area as old belt,it will have marks on drum from where original belt was on drum) then put rest of belt over mtr pulley and then idler pulley, turn drum by hand to align belts and put it back together in reverse order

OK, this one's a few years old. First, of course, disconnect power to the machine. Second, remove the two phillips head screws, one at L and one at R near the bottom corners of the front panel. Third, pull the bottom of the front panel toward you and unhook it from the dryer's top. Swing it to the side. You do not have to disconnect the wires to the door switch. Fourth, remove the four 5/16 hex head screws holding the drum supports and set it aside. Fifth, put belt around the drum. The smooth side of the belt touches the drum. Sixth, belt goes to the motor drive pulley and then around the belt tensioning idler pulley. Seventh, put the drum support part back on and turn the drum by hand to make sure the belt tracks OK and has no twists in it. Eighth, finish reassembly and test. Note: genuine Maytag belts usually have installation instructions on the back of the package, with a couple diagrams.