Surface prep, correct flux and brazing rod are important to do a good job. Not much more different from OA welding in terms of two handed hand-eye coordination. Choice to make as to if you will use silver rod (flows easily into lugs, works at lower temp but the actual silver rod is expensive) or to use bronze rod which with which you are more likely to overheat the tube. Excelent source of great products for either is;
http://www.cycledesignusa.com/

Before you take a torch to your frame tubes & lugs, get some strait gauge 1' and 1-1/8" tubing with 1/16" walls so that you can make a bunch or rings that slip over eachother and practice your technique, get a feel for the heat requred and do some destructive testing to confirm good penetration of the braze metal throughout the joint.