What's the best way, legally of course, to get the ball to reverse swing?

Hey Glenn
Excellent question. I’ll preface this be saying I’m not a bowler but from what I understand of reverse swing it comes from keeping one side of the ball shinier than the other.
It’s important everyone in the team is fully aware of what side of the ball they’re to be working on. In fact, that’s almost impossible as someone will always shine the wrong side. Probably better to leave it with one or two players to protect the ball.
Legally you can use sweat on the ball so make sure any sweat is going to the side of the ball you are shining.
If the ball starts to reverse, it obviously is an added benefit to the bowling team but only if the bowlers are pitching the ball up to give the ball an opportunity to swing.
I’ll leave it to others to provide technical advice.
Best of luck