Welcome to the exciting world of making a difference in another's life! What you are about to experience in tutoring young children is that they are a lot smarter than you think. Even though they are not quite reading, they are ready and willing to learn everything they need to start.
My first bit of advice is to not set your standards too high. I thought that after I was done the kids would know all their letter sounds and identifications. But, looking back on my notes, that is not the case. Sure, some kids made it to 'L' in the alphabet but, a majority only got about 5-7 letters down perfectly. Do not get discouraged! That is still really good and the tools they need to learn future letter sounds.
Also, try a variety of strategies when teaching anything. Each student is different and maybe the one thing you did not try might have worked. I learned that the hard way. And do not forget to take notes on what worked or did not work for each child. When you have notes, it is easier to make a lesson plan that fits accordingly to everyone's needs, including yours. It also may help to ask the teacher ahead of time to see if he or she has found a good technique with a particularly hard student. That way, you do not have to go through techniques he or she has already tried.
Good luck! You are going to have a most memorable time. Just remember to have fun with the kids, it really takes you back.