The text of the GPL offers a good description of the mechanics of applying a license to a piece of software. My quick checklist for applying a license includes:
Make yourself or the FSF the copyright holder for the work
Attach and distribute a full copy of the license with the source and binary by including a separate file
At the top of each source file in your program, attach a notice of copyright and include information on where the full license can be found
If your program runs in an interactive mode, you should write the program to output a notice each time it enters interactive mode
It might be helpful to include a “copyright disclaimer” from an employer or a school