Walkthrough

1. Yawn Up a Storm Request Guide
2. After relaying the information about Onix to Professor Tangrowth, talk to him by the ruined Pokemon Center to discuss what to do next. Ultimately, you'll settle for finding a Slowpoke than can change the weather by yawning.
3. Go back to the fishing dock area near the lighthouse and create a Seaside Tall Grass habitat to attract a Slowpoke.
4. Once you get the attention of Slowpoke, you'll discover that they are slow-moving, and require a clear and easy path. Start by crafting a few wooden stairs and set them up.
5. Back in the original area, moisturize the dirt road so it's easy for Slowpoke to walk on. Be sure to match their pace and speak to them during their breaks!
6. Lead the Slowpoke back to Professor Tangrowth and speak to them to complete the request and begin the important request, "Yawn Up a Storm".

Walkthrough source: Game8.