There are 2 ways that I've done it and both work.

#1 is to support the liner bead while the pool fills and then do it - down side of this is that the liner 'over stretches' and weakens.

#2 is to seal the back of the liner at the stairs and wait until the wrinkles are all gone and install the faceplates 'dry'. This is riskier because you cut the liner before any water is in it and, once cut, the stair section can't be moved. If you need more info, just ask