There are a couple of ways to start off while stringing a tennis racquet. The way I use is called the Double Pull method. All you have to do is:

• String the first two mains, (whether starting at the head or throat, it doesn't matter) and pull both strings at once in the tensioner.
• Clamp one string right up against the frame (head or throat side, whichever you start at) and release the tension.
  
• Then, pull the opposite string that you just clamped, and your off!

   This method works whether you are using a drop-weight or an electronic tensioner, and is a very easy way to start. The point of using this method is just so that you can have a straight string to clamp on easily as the beginning instead of a floppy, limp one. You re-pull both strings eventually, so the overall tension is the same as it would have been had you not used the double pull.