Slowly and carefully. It takes heat well, but it'll go from brown to charred pretty quick. I always use oil and spend quite a bit of time making sure the heat penetrates deep into the wood. I've used an open fire, stove, and my heat gun effectively.
If you can, heat a little reflex with dry heat. The wood loves it.
I made an 80#@26" flatbow that was 2" wide at the fade, tapered at twenty inches to 1/4" stiff tips, was about seventy inches long and completely flat after taking an inch of set to lose its inch of reflex.