Go to the "Create A Fighter" mode and select "Load Fighter".

Highlight the fighter you wish to max out, and press Y to preview the fighter.

Once your fighter loads, press X to bring up his stats, then press X again to hide them.

After this, press A to load your fighter.

If you go to edit his Fighter Skills, you'll noticed that your previous stats remain the same, but you've been given an extra 3024 points to put into your skills.
Distribute them as you see fit, then go to the menu where you can select "Finished".

Once the game autosaves, you can then go back to Create A Fighter mode and repeat the process until every skill is at 100.

Please note that this only works for fighter skills. Your Strength, Speed, and Cardio attributes are not affected by this glitch. Also, this does not work for Career mode.