Your design project is complete and you want to show it to your client or on your own portfolio page.
So far, you may have used screenshots to design your presentation. Or you have brought your notebook with you to your client, whatever is problematic.
But fortunately there is a much better solution!
What you need is Photoshop and some knowledge of working with layers. There are a lot of great mockups that, based on Photoshop, show your work in a beautifully designed environment.
Here are some examples and a (never perfect) list of free mockups based on Photoshop.
Samsung Galaxy S8 Concept Mockup Sketch freebie - Download free resource for Sketch - Sketch App Sources
One more thing you can design for your presentation
also if you need to present some print examples just find the Photoshop Mockups to build.