Instructions
Talk to the guard on the first floor of the radio tower near the stairs. He'll tell you he only lets people by who are wearing Team Rocket uniforms.
Exit the radio tower and head to the right. Enter the underground pass using the entrance next to the Name Rater's house. The other entrance past the casino and near Bill's house is fine too, but this one is closer.
Walk through the now-empty underground pass to get to the photo booth station. Normally, there are trainers in your way, but right now they are nowhere to be found.
Look for the Team Rocket member standing near a potted plant. If you walk in front of him, he'll notice you and act as if he wants to battle, but he's not interested in that right now. Instead, he mistakes you for a new recruit and allows you to change into a Team Rocket uniform.
Scroll through the additional conversation that follows. The Team Rocket member warns you not to talk to townspeople while wearing the outfit, because you'll scare them. If you talk to any of them wearing the outfit, they'll deplore you for being part of such an evil organization.
Leave the underground pass the same way you came, and return to the radio tower. If you're not prepared for battle, you should stop by the Pokemon Center first and make sure you are.
Talk to the Team Rocket guard. He will still be near the stairs on the first floor. Be prepared to battle a trainer with Pokemon leveled at around 24.