Glue the gasket to the pan with RTV.
Put some plastic wrap down on floor.
Put gasket and pan on plastic wrap.
Add some weight to pan (stacks of old newspaper work)
Let sit overnight.
Install pan with RTV smeared on gasket to tranny.
If you don't let the gasket and RTV dry onto the pan, when you tighten the bolts the gasket tends to be pushed out from between the pan and tranny........(RTV makes a great lubricant if it isn't dry).