A bow is just like building a weapon that has two blade ends, with the handle in the middle.
The only real other difference is the bend. There are several ways to bend PVC (and you should be using PVC, there really aren't any other cores that will bend). My favorite way is over the stove.
Mark where on the pipe you want to bend with a sharpie.
Turn a burner on your (electric) stove on high, and wait for it to get hot.
Gripping the pipe a little ways from the mark (you don't want to get burnt!) slowly rotate the pipe like you're roasting a marshmallow (rotate it from both sides at the same rate so it doesn't twist on you!)
When it starts to get soft, remove it from the heat and put it on the floor. Bend it without twisting to the angle that you'd like. I then rub it with a cool, damp cloth to get it to cool faster.
Once it's cooled so that it is no longer mailable, run some cold water over it so you can handle it.
Do the same series of steps to the other bend.
Some tips:
I use a floor that has a pattern (like checker boxes in the linoleum) to make sure I've bent the two sides the same amount.
When bending the second side, make sure the first side is flush to the ground, or else your bow will bend funny.
If you're not using an electric stove, the above isn't the best method (though it should work). Make sure the pipe doesn't touch the burner, or it'll get.. er, burned. That's bad.