Every graphic designer (big or small) needs a dope Photoshop mock-up in their collection of dodads where they can display their professional graphic work properly. If you have a high-resolution image of a laptop or similar device and a screenshot of your design work, you can combine the two using Adobe Photoshop and BOOM! You got a mockup. This helps clients see your work in the right context, on a device that looks real. The more realistic you show your samples, the more accurate they can gauge the vision!
Now this tutorial is for mockups that uses device backdrops. In this one I will use a iphone however, the device can vary.
Start with a backdrop of a laptop or mobile device. Get an appropriate high resolution photo on which can represent your website snapshot properly.
Open the picture in Adobe Photoshop. File > Open
Use the rectangle tool (Keyboard Shortcut U) and draw a rough rectangle roughly the size of laptop screen.
Right click on the new layer called Rectangle. Select Convert to Smart Object.
On the top tool menu, go to Edit > Free Transform Path (Keyboard Shortcut Ctrl+T)
On the top tool menu, go to Edit > Transform > Distort.
Move the edges of the rectangle freely to match the tilted laptop screen. Click and hold any edge of rectangle and move it to match the related edge of the laptop screen.
Repeat the above step with rest of the three edges to match all the four rectangle edges exactly with the laptop edges. To get better results zoom the image to match the edges.
On the rectangle layer, click on the small Smart Object icon on the bottom right.You'll see that a new tab will open and there will be an plain image in front of you.
Take a screenshot of the website you want to display on laptop screen. Then paste it on the above mentioned plain image. You may need to adjust the screenshot on the rectangle shape plain image.
Save the image (Ctrl+S) and close the tab. And it's done!
You'll see that the screenshot has perfectly overlapped the existing image and your website is ready to be presented to the whole world.
Remember there are ready made mockups for free where all you need to do is replace the smart object. Some are free and some cost money! Choose your method based on your means and flourish accordingly.