There is no set way to catch the regi's (it took me a long while) it all comes down to luck. I can give you a idea of how to get them though.
First Stock up on Timer Balls and Ultra Balls.
Next make sure that you have at least 2 pokemon in your team that knows False Swipe (always leaves the pokemon with 1 Health Point) and also one or two that know a move that puts the oppsing pokemon to sleep.
I think the next part is obvious, get them down to 1 health Point put them to sleep then throw tonnes of Ultra/Timer balls at them untill you get them, could take a while though so be prepared.
Of course if you have a master ball you could always use that XD. Hope that helps you in some way