There are really only three requirements for a power of attorney to be legal:

1. You must be able to understand what you are doing;
2. It must be in writing; and
3. It must be notarized.

Witnesses to the power of attorney are not required.  But, because you should be very careful about what power you give someone else, you should consult an attorney about exactly what should be in the power of attorney and how it should be written.