The most important thing is to know where your bend starts. This is tough to nail down, but start by putting a piece of tube into your bender. There is a set screw on the main die u-strap. Tighten it against the tube to keep it from moving anywhere. You should NOT be lubricating the tube or any of the die or backing block surfaces on this type of bender. Put a bend into the tubing at least 5 degrees. Look closely at tubing from the side and note where it starts to neck down. The beginning of the bend is just before it necks down. This is the last part of the straight section of tube with the original outter diameter of the tubing. MARK THIS LOCATION ON YOUR DIE.
Once you have that figured out, continue to bend a 90. This type of bender should be cable of doing a 90 even after a little springback. Make sure the piece you bent is exactly 90 degrees. Then you can find the start and stop points of the bend the same way I mentioned above (just where the tubing starts to neck down) and use this piece for layout purposes.
Nailing down the start location of the bend on the tube and marking it appropriately on the die is very important. Also, if you are using the Pirate4x4 Tube Bending 101 article as a guide, it's also critical to mark these correctly on your dummy piece of 90 degree tubing.
One thing you may find hard to deal with is springback. My advice is that after you have finished pumping the jack on the bender and you think you are at the right angle, release pressure on the jack but leave the tubing in the die (do not loosen the set screw on the u-strap). Measure the angle of the tubing to make sure you have your final angle correct. If you do not release pressure on the tubing, you will not be measuring the sprung back angle the tube and your part will be the wrong angle when you take it out of the bender. If you have to remove the tubing from the bender, be careful to realign your start point with the start point marked on the die.
Once you've got that figured out and you've laid out your part on the floor, start at the center and work your way outward.
A helpful forum with a lot of knowledgeable tube bending folks is
www.offroadfabnet.com If you are still having trouble, you might try posting over there too.
