Twisting the string is no big deal, you just take it off on end and twist it. Just look at the string and determin what direction you need to twist it, one way will shoten it and the other will make it longer.

what type of string do you have? If it is fast flight it should not need to be twisted all that much. You should get a book (Becoming the Arrow is a good one) with some tuning info in it.