Getting Started in Yo-Yoing
Beginner Players: If you would like your fixed axle yo-yo (where the axle does not spin independently like a ball-bearing axle does) to come back up automatically, you can double-loop the string around the axle.To make a double loop on the axle you pinch the yo-yo string about 3 inches from the yo-yo, untwist the string until you can open up the loop with your fingers, twist the string once and then put the loop over the yo-yo again. It’s the same way you would put a rubber band around a deck of cards. Once you have the double loop on the axle, just retwist the string and you are ready to go. Just remember that in order for the yo-yo to sleep (to do tricks like Walk the Dog) you will have to undo the double-loop, and go back to the regular single-loop.
String Length: One of the most important things for yo-yoing is having a string that is the right length. In general, when the yo-yo is unwound and sitting on the floor, the string should come up to your belly button. If your string is too long, let the yo-yo unwind and rest on the floor, then cut the string where it meets the bottom of your sternum (the bone in the middle of your chest that all of your ribs connect to). Next, tie a solid loop on the end. The string should now be the right length for you. Some people like the string longer, and some like it shorter. Do whatever works best for you. If you’re happy with the string length right out of the package, that’s great.
Solid Loop and Slip Knot:

The Slip Knot: Once you have a solid loop on the end of the string, you need to make a slip knot so that your yo-yo won’t fly off your finger and kill anyone.
To make a slipknot, open up the solid loop at the end of the string, come down about 5 inches and grab the string, fold it over, push the folded part through the original solid loop, pull it all the way through, and put your finger in the new loop you just created. This is your slipknot, and it goes in between the two joints in the middle of your middle finger.
String: You should always use a string that is in good shape. It will help the yo-yo play better, and will again reduce the risk of the yo-yo flying off and killing someone. Besides, strings are way cheaper to replace than a broken yo-yo. It is time to change your string when it starts to get all dirty and nasty, when it starts to fray, or when it just doesn’t work as well anymore.
Replacing a String (with Dazzling Dave Schulte):
The most important thing is don’t give up! With enough practice you will be able to do any trick on this site.