dirt simple
there should be a decal on the deck that shows the belt run from above
Failing that the actual belt run is usually depicted in the parts book.
As far as installing you run the belt around the pulleys except the one next to the tensioning arm.
I am not sure if you mower has one but most will have a square hole in the tension arm which is there to put a socket wrench or breaker bar into .
You use that to stretch the spring to give enough slack to slip the belt over the final pulley usually the left,( non discharge ) side .
If there is no square hole some times you can do the same using the nut on top of the idler pulley and as a frail old widow showed me, even a ratchet tie down strap .
If the belt has been sitting around for a very long time it may have taken the folded shape which can make installation a bit harder but if you unravel it and sit it in the sun stretched out ( tent pegs . old batteries end of dog lead etc ) for a couple of hours it will return to the proper shape.
Yo save space some belt retailers will fold the belt 4, 5 or even 6 times