How did Pepper use the Rescue armor in "Avengers: Endgame"?

This was probably not her first time using the armor

Pepper is a smart woman. She would have known that as soon as Tony got involved with the Avengers again, there was a chance that things would go wrong and they would need help ('rescue', if you would). It's unclear how long it took to develop and build the time travel device, but it was likely several months at least given the complexity of the project. Plenty of time for Pepper to practice with her suit of armor.

Tony is constantly redesigning and improving his suit designs - that includes the control interfaces

Tony's original suit (the one built "in a cave, with a box of scraps") was controlled entirely by feedback from user movements. His next iteration was largely designed the same way, but augmented by integrating J.A.R.V.I.S. into the control systems.

Since then, Tony has spent a ton of time refining the suits' design. In Infinity War, his personal suit is no longer controlled mechanically at all, and is instead linked directly to his brainwaves, giving him the ability to control his suit with the speed of thought, and allowing him to achieve many feats with his nanobot suit that would be impossible to design into a more mundane control system. It seems unlikely that the suits he makes for other people have the same level of control, but even a small bit of biofeedback could greatly enhance the suit's control.

Pepper's suit undoubtedly has a great deal more stabilization and autonomy than Tony's original suits had, making it much more user-friendly and needing a lot less practice to use effectively.