Heat the work to forging heat. Within the limitations of your forge, try to heat the area to be bent evenly, without heating up the parts you don't want to bend. This sounds difficult, but in reality, just do the best you can. The more evenly you can heat the bend, the easier it will be to get an even bend; the less you heat up the parts that you don't want to bend, the easier it'll be to avoid undesired bending.
Then grab the ends on either side of the bend and bend away. With practice, you'll get a feel for how to apply the right kind of torque to put the bend where you want it.
Depending on the size and shape of the work and how it gets heated up, you'll want to try different ways to grab the stock. Tongs are most common, at least for one end, but for very long pieces, where one end is cool, it may work better to hold it directly. You'll also use vises, a bending fork hardy tool, or the holes, horns, or edges in your anvil. It often helps to use the hammer to tap the bend a bit to make the bend conform to your intention.
For twists (just another kind of bend), an old-fashioned monkey wrench is handy, if you find one at a garage sale. Welding a handle to the top of the wrench the same length as the handle it came with will make it easier to apply just twist, with no bending torque.
You can also unbend the bend you just made and try again. Cool the part of the bend you like, so it stays put, then you can bend the part that's still hot some more. You can bend steel that's cooled below forging heat, but generally you'll only get one bend before it's work-hardened too much for another. If you push it past that point, it's apt to break on you.
Bending over the round horn of an anvil:
Next Chapter: Project 1: A Simple "S" Hook