First, go to the shipyard in Slateport. Talk to the man that you first come across, who seems to be staring at a piece of paper. He tells you that Captain Stern is at the museum.
Go to the museum at the east of town, then talk to Captain Stern on the second floor. He'll take the goods and arrange to leave. Before he can move, 2 members of Team Aqua will come in and try to steal the goods. Battle them, and when you defeat them both, the Aqua leader will come, give you a brief warning, and leave.
Edit: punctuation, grammar, and clarity improved
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You go to slateport and you deliver the Devon goods to captain stern.
You can either go to the next route or get the exp.share from the president.
Capt. Stern is not in Pokemon Leaf Green. He is only in Pokemon Ruby, Saphire or Emerald.
You deliver Devon's Goods to Captain Stern who is in Slateport City's museum.
In the slateport city boat dock place