Walkthrough

1. How to Change the Time
2. After making rain possible in the Withered Wasteland, speak to Charmander about their concern of losing the flame on their tail when they get wet.
3. Get a leaf den kit from the PC shop and find somewhere to set it up.
4. Gather the necessary materials, call the necessary Pokemon, and begin the construction. You can advance the time in your system settings to instantly finish construction.
5. Place 3 pieces of furniture inside the den, then invite Charmander to move in.
6. Once you're done, go outside and speak to Charmander again to complete the request.

Walkthrough source: Game8.