The smart trick is to drive the scharp point of an ax along a line marked on some piece of sheet metal. Only the sharp point has to cut through. To let the blade sink deeper in the cut spoils the work.
The line can be straight, can make curves or sharp angles. It only takes a little practice to follow the line with the sharp front end of the cutting edge. Fast but light hammerstrokes on the flat side of the ax force the cutting.
The edge of the cut sheet bends inwards. This can be flattened, but folding it over some more, then flattening the double layer, makes the edge stiffer, soft and safe for handling.

When an 200 liter oil drum is used to recuperate the sheet metal, the cylindrical shaped side part has to be flattened. This works well by pounding it with a log as shown in the picture. Start in the middle and then works towards the ends.

Folding sheet metal is easy using almost the same method as with cutting. Hammering the ax 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 hammerblows 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 ax.