Look up your dryer by manufacturer and the serial number, go to a parts website (not Amazon you'll get burned) and order a repair kit that includes the pulley, tensioner, belt, and rollers. Watch a few YouTube videos, some of the repair sites have them on there, and you're good to go.
You can do this yourself but it's so much easier to put the drum back in with a second set of hands. Also, you'll probably want to have a shop vac handy because you'll wanna clean all the lint from inside since you have it open anyways. As far as appliance repair goes this is about a 3.5/10 on a hard scale. If you can turn a screwdriver, understand directions, and have a bit of strength for that belt (not a lot just some) you can absolutely do it.

