Mockups in PSD format are available on TutKit.com for use with Adobe Photoshop CS4 and higher. To insert a design, all you have to do is open a mockup and insert your design into the smart object contained in the mockup. This is how you proceed step by step:

1. Open a mockup in Photoshop.
2. In the Layers panel, you will find at least one Smart Object layer highlighted in green. Double-click on the Smart Object thumbnail to open a PSB file.
3. Blend out the example image in the layers.
4. Go to File>Place and embed...
5. On your computer, select the design you want to insert and click on Place.
6. In the Open as smart object window that opens, select Media frame from the options under Crop to.
7. Click OK. Your design is now in the smart object.
8. Save the PSB file (or the Smart Object) with Ctrl+S.
9. Now close the PSB file with Ctrl+W - your design is in the mockup.

In some mockups, you also have several smart objects at your disposal, into each of which you can insert a design. In addition, many mockups offer you prepared backgrounds from which you can select the background that suits you best. Of course, you also have the option of inserting your own backgrounds.