A couple of methods:
For a gradual curve like a cowl side, make a templet from a piece of 3/4" plywood. Mount the plywood to a table, clamp the tubing to your form and pull cold. Sometimes you need to correct your curve templet to make up for spring back.
If you need a sharp curve like the radius at the cowl corners, pack the tubing with dry sand. (DRY! use the oven to heat the sand to dry it) Cap the ends, Make a form from flat stock tacked to a steel table. Heat the tubing and stretch it very slowly. This will work 3 times out of 10. With practice you will learn the limits of the tubing. Some times it helps to build a square tube shape tacked to the table to help keep the sides from buldging. It's still hard to keep the outside from sucking in.
Also, stated you can cut slits in the tube and re-weld them, grind them etc.
Also, cut three sides off the tube, wrap your tube around your templet, cut curved side pieces out of flat stock and cap the outside radius, weld and grind.
Or make or buy a bender. These most always crush the inside of the tube as they form the bend.
Good luck,