Question: how did usopp learn haki?
Answer: Usopp has awakened his Kenbunshoku Haki during the latter half of the Dressrosa revolt, as he was able to see auras of Luffy, Law, and Sugar, who were in the royal palace, from the old King's Plateau near the Corrida Colosseum. With this, he was able to pinpoint Sugar's location and snipe her down from such a distance.