phase 1: make sure you have the dlc suits (don't know if the launch version had extra skins)... it is VERY important to have extra skins to use
phase 2: .......................
phase 3 is profit
phase 4: press start on the title screen, choose your game file, then on this screen where you would choose to enter into your game, press Left, Left, Down, Down, Left, Left, Right, Up, Up, Down on the d-pad, you should hear a confirmation noise after the last "down" (note: this is irrelevant once NG+ is available because then skin choice freely available)
phase 5: when you enter your game you will be given a screen with all the skin choices before actually entering the game
phase 6: enjoy the skins (again, assuming you have those skins available in the 1st place)
beyond that, i cannot help and that's as descriptive as i can be about this. i tried to keep sarcasm to a minimum but as you can tell, this was not a difficult thing for most people