It's easy to me:
1. Scribe a line across the beam center section longer than the section you're about to cut out, I normally cut about 6" out.
2. Using a sawzall, carefully cut the center section out as straight as you can. I can normally do this pretty well because my eyeball is pretty well calibrated. Some people use other methods like a pipe cutter. You don't want to have a wide cut so don't use a grinder. If you do, use a cutoff wheel (thinner). ALSO, ONLY CUT ONE TUBE AT A TIME!!!! That way the other tube keeps you in alignment for the beam.
3. Clean all the metal bits out, attach a piece of angle iron to the back of the beam to hold the cut out piece steady for tack welds.
4. Rotate it 1/2" to 5/8" for a race car, finish weld it, nice and hot!
5. Repeat on second beam tube.