You have to put a thin film of grease on the crankshaft so the seal won't be torn out of place when you start the engine.
On a single piece seal, do the monkey shuffle to get the seal over the flange. Let it have considerable time (I use a day) to recover shape. Then, a really thin film of grease on the crankshaft - only where the seal will be. Then, seat the seal with pliobond. Another day to cure, and it should be good to go.
Can be a little tricky to get the grease on the shaft yet keep everything else clean for the pliobond to set up.
The seal should be fairly tight.