Instructions
Create your secret base by going to any landmark that has a square indentation in it and using the move Secret Power. You can only create a secret base at a rock wall, a large bush or a tree.
Collect any Pokemon decoration you wish to use in your base. A complete list of the decorations and their locations can be found online at Serebii.com.
Go to the PC in your base and select the "decorations" option. Select the decoration you wish to display in your base and press "Confirm." The decoration will appear in your base. You can only have a certain number of decorations displayed in your base. The exact number of decorations depends on the size of your secret base.
Remove decorations by selecting the "decorations" option from the PC and selecting the "Remove" option.