How to butt weld 2 pipes together straight?

The angle iron as suggested will do the best job for you. Don't butt the pipes together tight because they'll warp (pull) when you weld them. Instead keep a slight gap between them about a little less than the diameter of your weld rod or wire. Tack the pipes in 4 places with short tack welds then remove the angle iron and finish welding alternate quarters (between the tack welds) and the pipe won't warp crooked. Depending on the wall thickness of your pipe it may take two or three passes to fill the gap. The first pass is called the root pass, the second the hot pass and the final the cover or cap pass. Clean each pass with a wire wheel or brush before each subsequent pass. Helps if you bevel both ends of the pipe slightly before you tack it together. In the field we used a device called a "Jewel Clamp" which was a glorified piece of angle iron with chains and threaded tightening screws that held both ends of the pipe together until the tack welds were in place.