Folding sheet metal is easy using almost the same method as with cutting. Hammering the axe along, but prevent to make a cut, only make a linear sharp dent in the sheet. Bend over the part that has to be folded. Sometimes it helps to fold over an edge, but with some practice just applying some force and hammer blows does the job.
A nice property of this method of folding is the accuracy of the place where the sheet folds. You get it exactly at the line that is dented with the axe.