The smart trick is to drive the sharp point of an axe 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 hammer strokes on the flat side of the axe force the cutting.
The edge of the cut sheet bends inwards. This can be flattened, by folding it over some more, then flattening the double layer, this makes the edge stiffer, and soft and safe for handling.
A drum has strong edges made to shape the connection between bottom and cylinder. Good use of this reinforcement is made by cutting close to the edge. Cutting with the axe is easier when the sheet metal does not bend away from the cutting.
But cutting free suspending sheet metal is also easily possible by driving the axe with more horizontal hammer blows at the same time inserting the tip of the axe at a sharper angle to the surface of the sheet metal.