Instructions
Travel to Floaroma Town at any time during a Friday. Head east of the town, via Route 205. Walk along the route until you come across Valley Windworks. You know you've found it when you see a field with windmills.
Head up to Valley Windworks to find a Drifloon floating around near a cluster of three windmills, next to a sign. Walk up to the Pokemon and press "A" to engage it in battle.
Battle the Drifloon. If you happen to have a Quick Ball, throw it right away to catch the Pokemon without fighting it. Otherwise, take advantage of Drifloon's weaknesses and capture it. As a Ghost and Flying type Pokemon, Drifloon is weak against Ice, Ghost, Rock, Dark, and Lightning type Pokemon. Once you have taken away enough HP, Drifloon turns red. Throw a Pokeball to capture it.