When you freeze them, you freeze the piston with the seals on it and the seals expanded or compressed (whichever way the seals lip goes) so that it just drops in and then the seals expand out to the drum when it warms up.
The easiest way is to take something like a .010 feeler guage blade and taper it to about 1/4 wide at the end.
Sand, mototool or whatever the edges all rounded and smooth.
Put the seals on the piston and wipe them down with vaseline or assy lube made for transmissions. (both pretty much the same)
Lube the inside surfaces of the drum where the seals slide the same way. (you almost can't get too much on them)
Lay the piston in position and with a light downward pressure on the piston, put the feeler guage tapered tip between the drum and the seal and slowly work your way around the piston with it.
You should feel the piston giving as it begins to go down into the drum.
Alternate between the inner and outer seal frequently, so they both get tucked in as evenly as possible.
Just in case:
Keep in mind that all the seals should have the sharp edge of the lip facing the inside of the drum.
Finally found a pic of how a feeler guage is used, but I don't think he tapered it.
It's a lot easier and safer for the seal if it's tapered and shiney smooth on all the edges.