My understanding is that to make mosiac damascus you's weld it in a "can", usually square tubing. When in the oxygen free can you don't need any flux. Close up and weld one end of the tube, insert your solid pieces for your design, fill in the spaces with powdered steel, pack it down and put in more powder, then close the other end end weld it shut leaving a tiny opening so gasses can escape without exploding the can. Some people put in a few drops of WD40 so it burns any residual oxygen out. The hard part isn't making the damascus, it's keeping the pattern nice and even, which is why alot of mosiac damascus makers use a press or a power hammer.
By the way, I've never made mosiac damascus, just read alot about it, so take this advice with a grain of salt.