1. Go to the area north of the Pokemon League building between Snowpoint and Sunnyshore.
2. Enter the water there.
3. Surf around until you find Lapras.
4. Weaken it a little (don’t knock it out) and throw an ultra ball at it until you eventually catch it.
5. Or you can bring a Pokemon that knows Dive and dive below the water. Once you see Lapras, resurface to start the battle.