I've got a 2-foot long piece of 2x4 that I use for straightening aluminum arrows. I used a rat-tail file about the same diameter as an arrow shaft to file a shallow groove
Lay the 2x4 flat. File a groove on the top surface of the board at one end (groove should run parallel the long axis of the board). When filing the groove, hold the file at a very slight angle so that the groove is about 1/4" (or maybe a little more) deep at the end of the board and runs back about 3" from the end of the board, where it tapers back up to the surface of the board. Be sure to make nice straight strokes with the file, so that the groove stays pretty straight and true. That's a pretty long and possibly confusing way to describe a very simple groove, but it's the best I can do without pics.
The reason for the groove in the board is to distribute the bending stress around the girth of the arrow somewhat (as opposed to just bending it over the flat sharp edge of a kitchen counter or workbench)-- you'll be much less likely to put a kink in the shaft if you bend it over the end of this grooved board.
Just lay the shaft on the board, with the apex of the bend right at the end of the board (and in the groove). With the heel of one hand holding the shaft flat to the board (just behind the 3" groove), use the other hand to push downward on shaft to flex it back in the opposite direction of the bend. Go easy until you get the right amount of pressure/bend applied to the area so that it straightens.
It's similar to straightening a wood arrow-- just keep working the area until it's straight.