Begin by opening the acetylene valve on the torch very slightly and igniting the torch with a striker. The flame should be fairly small and sedate with a smoke of heavy black soot If the flame is larger, close the acetylene valve slightly before continuing with this process. Now, open the acetylene valve slowly, increasing the size of the flame. A little bit of valve movement goes a long way here. Continue increasing the amount of the acetylene in the flame until the black soot becomes less pronounced. Unless you are cutting very thick materials, the flame should not be particularly large. If the acetylene flame at this stage is larger than about a foot long for relatively thin materials, say 1/8", then you are probably using too large a tip, setting the acetylene pressure too high on the regulator, or simply opening the acetylene valve too far in this step. One of the most common problems people have in learning to torch cut is using too large of a flame. Remember that you are oxidizing the metal, not melting through it. Use the smallest flame as you can get away with while still being able to preheat and cut.