Using real rivets is best but as already pointed out, time consuming.
The way I cheat is using a centre punch with a rounded point , not too sharply pointed. You can use a piece of steel rod or bolt of the appropriate diameter, ground to a rounded point. Another punch is needed, and this is the most important one, it has a concave hole on the end instead of a point ( I am not sure what it is called). Scribe a line where the line of rivets are to go and determine the best looking distance between the rivets, and mark the place of each rivet. Lay the sheet metal on a piece of soft wood, place the center punch where the rivet is to be and give it a sharp tap with a hammer. Continue along the line. You will notice that this makes the sheet of metal warp, don't worry, the next stage counteracts this. Do one line at a time. When the line is finished, straighten out the sheet as best as possible and turn the sheet over, put it on a harder surface.
Grab the concave punch, place it over each "pimple" that you made and tap with the hammer, best to try softer blows first, do this along the line, this should make the "rivet heads" rounded and more defined and you will see that the sheet of metal straightens out pretty nicely as one progresses. When the rivets are finished the line of dents on the back of the sheet can be hidden by filling in with soft solder using a soldering iron, and then sanding it flat. If the sheet will not be getting hot eg. the side of a tender, then car body filler will do.
I don't think I need to say this (but I will

)... try it out on a piece of scrap first to get the hang of it. When using the concave punch, do not tap too hard or you could end up with small circlar indentations on the sheet around the rivet head. This method works great with flat metal sheeting. Preformed tanks etc. would probably be more tricky, and I think the warping effect of the first stage might be difficult to straighten up.
I hope this was useful !