After the tubing is cleaned up and ready to weld, tack each end so it doesn't move. Then, if possible, stand the joint up so you have access to laying down two beads. Start at one end of the tack weld and move to the other end, making small circles or curlicues. Be careful with your settings. For welding tubing, you'll probably need to turn down your wire speed so the puddle doesn't become too big.