For any Topre board, I've always been putting the plate (with sliders and all) upside down, and then align the domes on each slot, followed with the springs. After I, as gently as possible, drop the PCB on it and then put on a few of the screws, plug it in and see if I messed up. If I mess up (happens more often with Leopold + BKE), I take it apart and try again. It's a bit tedious sometimes.
I find the easiest way to do this is to remove the keycaps on either sides of the board and then use two heavy boxes to prop the keyboard enough so that none of the keys are pressed when you're aligning the domes.
I feel like doing it the other way would be much more difficult because after you drop the plate on the PCB, you'd have to somehow either put on a few screws while it's still right side up, or flip it over without moving any of the domes.