To add your own images or designs to a smart object layer, go to your Layers panel, and double click on the white rectangle thumbnail on your smart object layer. These layers are usually labelled 'Add Image Here' or similar.
Double clicking on your layer thumbnail should open up a new design tab in Photoshop. Here you can place your images (File > Place Embedded) or simply create your own design within this workspace. Once you are happy with your design, click the X icon on your tab to close it, and select Save.
Your design should now be applied to your original PSD file, and allow you to play around with your photo effects until you are happy with your results.
You can check out all of our Photo Effect packs such as the Watercolor Creator, Vintage Watercolor Photoshop Effect and Watercolor and Sketch Photo Effect in our Marketplace,a nd if you have any queries, our Support Team are always here an happy to help!