I turn the keys upside-down on the table then put a small square over the '+' hole, then push in the peg.
On the Leo, perhaps you can bend the tiny square over the peg while it is in the board with your fingers, then push the key over it.
Must be very careful though, because it becomes VERY tight on my keys. No problem when you remove the pegs with a screwdriver, but on the Leo you would have to rip it off while the stabilizers are installed, and that would put a lot of force on those little parts.