Depending on the size, the best way to do it by hand, would be to make a jig and use a hammer or mallet. If its small enough you can clamp it in your bench vise at your bend line, make sure its square, and hammer it down til its bent to the desired angle.
For larger pieces, You could go get a couple lengths of 2x2 angle iron, and clamp them to the edge of your work bench or another sturdy place and bend them like that. Take one piece of angle and put it on the edge of the work bench, like a long corner cap, Then take the other piece and face it the opposite direction, with your sheet metal between them, calmp it securely with good clamps & whomp accordingly lol. You could prob also use wood instead of angle iron, but the angle iron would make for a more concise and effective make shift break. Also, if you're needing a rounded bend, use pipe instead of angle iron on the bottom side of the bend and hammer the metal to the shape of the pipe.
The key to getting good clean bends is first off making sure you're measuring where the bend needs to be, and then making sure you have it nice and square with your break jig. And then making sure you have a good solid jig to hammer on and form your sheet to. If you have something flimsy, your metal is going to be allowed to bend passed where needs to stop depending on how hard you hit it which will likely result in an inconsistent bend. Alternatively, instead of using clamps, to make the jig much easier to deal with and much more solid, Drill some holes and put some long bolts in them, that way instead of fumbling with clamps, you can just loosen/tighten a couple of bolts. The just make sure you have them flush/square with each other and drill them at the same time so you dont risk being off on your hour drilling a bit. I would personally weld them at the ends, drill them, then cut the welds with a grinder.
And if you really want to get fancy, you could make an actual hand break very easily if you can weld. Just weld some hinge taps on each end of the angle, secure them with lock nuts & bolts. Wed you leaver on to the pivot piece, drill some screw holes in to the other to secure it to your work bench or whatever work surface, and bam, stick you some metal in, pull the lever and bend the metal. You'll probably save money and end up with something better and suited for your tastes than whatever chinesium you can find to buy. this way.
More than one way to skin a cat. Just gotta put your mind to it.
