Everything is very simple! All my Psd Mock-Up files have the same structure and having understood the principles of work, you will not have any difficulties! First of all, make sure that the downloaded archive is not damaged, extract the files and if everything went well you can proceed to the next step.
A new window will open the file of the smart object, for example there is already a design in the form of a layer "test image".
3 STEP - Insert your design!
4 STEP - Save the smart object by pressing the key combination CTRL+S. Close the smart object file.
Congratulations! Looks great!
Repeat the steps with other smart objects (marked in red), replace the background, open the group folder and adjust the brightness of the shadow, reflexes and light (change the transparency of the corresponding layers).