Try this:
I use the following tools: small hammer, chunk of steel (anvil), knife (since replaced with miterbox/saw), and a center punch/drift that has been filed and polished to minimize tool marks.
Cut the tube to the length you want, making sure both ends are square. I now use miter box and razor saw for this:
Once you've cleaned the ends up, stand the tube on end on a hard surface (a 1/2 thick chunk of steel makes a fine anvil), and set the punch in one end:
Lightly tap the punch with a small hammer untill you reach the desired flare:
And you will end up with this:
You can polish a length of tube to get a chrome-like finish before cutting, and to minimize the tool marks inside the flare, be sure to keep the beveled end of the punch clean and polished. If you want to make long injector tubes for drag cars, cut a piece of hardwood so the thickness matches the length of the tubes you want to make. Drill a hole the same size as the tube, then use a countersink to make room for the tube to flare, and repeat as above.