It takes a lot of practice, if you want to do it by hand and not using one of those molds or whatever they call them, it can be a bit tricky. This is the way I like to do it... Assuming the milk is nicely foamed, hold the cup(with the espresso) in the left hand and the milk jug in the right hand. Pour the milk in a thin stream into the espresso, quite slowly, until the cup is about half full. You should make sure you're only pouring hot milk and not foam, this depends on how you foamed the milk and how you're pouring, as I said it takes practice. :P When the cup is half full, position the stream further to the left in the cup and then you can start "shaking" although I would rather call it "swinging" the jug from side to side to bring the foam into the cup. While swinging, you should drag the stream of milk steadily to the right of the cup and, if all went well, a white and foamy ribbon-like pattern will emerge. To finish, lift the jug slightly and drag the stream of milk to the left of the cup again. This should create a design called a rosette. :) This mostly depends on how much foam is in the milk. If it's too little, the pattern will be pale and almost invisible, if it's too much the pattern will be too thick and become one big white splotch of milk foam. This will take dozens of trials and errors, I recommend starting slowly and steadily and learning from your mistakes, and most of all finding the most comfortable way YOU like to do it. :) I agree, this would probably make a great video if explained thoroughly.

They have some of those online... I could make one :-)