Like Karen said, you do not have to close the circle tightly. I usually do not begin to tighten it until I have done a few rows so it really makes the pattern more adjustable to compensate for tendency to cup in the middle.
Also depending on how many stitches you are going to have to put in the first row, you may choose to make the circle bigger. For example, if it called for 12 stitches, I would wrap it around two fingers. Or if it is thicker yarn.
You should try a few to get the hang of it. If you get the end tangled around around the circle, it may be difficult to evenly tighten the circle. I usually turn the whole thing upside down with that end facing me and put it on a flat area, and gently ease it through, I never tug on it.
Hope you like this method, I think they should begin all the patterns worked in round using this method.