After you build all 3 homes, Tom Nook will then task you with customizing a wardrobe. He will first give you a DIY recipe for a wooden wardrobe. Once you receive it, head to the workbench and craft it using 12 wood. Afterwards, talk to Tom Nook and select the option to talk about the wardrobe. He will then want you to interact with the workbench again. To customize the wardrobe, select the wooden wardrobe then a screen will pop up where you can select the color you want the wardrobe to be. Once you pick your color and have it selected, press + to officially make the selection. When your wardrobe has been customized, talk to Tom Nook again. He will then let you know that you will now have access to customization kits from the shop.