The Ilford 120 film has a gummed paper strip attached to the end. After the final frame you just keep winding until you hear it unhook from the first spool and then the "tick-tick-tick" of the end of the backing paper rotating in the takeup spool area (most 120 cameras have a tongue which keeps the film tightly wound). Open the back, carefully remove the film, lick the end of the paper strip and wrap it tightly around the film to hold it. Be very careful not to drop it before you get to that stage as if the film unrolls itself you'll expose the lot.

Try to only lick a small area of the strip - if you can leave the very end loose then it will be easier to rip it off again in the darkroom when you come to process the film, otherwise it's like trying to find the end of the roll of tape in the dark!