How do I ensure that I cook scallops all the way through?
High heat and quick cooking are essential for scallops. Several minutes cooking will result in fishy racquetballs.

Firstly, remove the scallops from the fridge half an hour or so before you cook them, so they're not stone cold (but don't put them anywhere warm or in the sun). Heat your olive oil in a pan until it starts to sizzle. Season the scallops, then place them in the oil. Don't move them for 90 seconds so they develop a nice crust. Once that's happened, turn them over and cook for another 90 seconds. That's it.

If you want to see the process, Gordon Ramsay's official YouTube channel has a couple of videos:

Part One
Part Two

(Part two follows straight on, so you can see how quickly they're done).