To make your own Johnlock manip, you don’t need Photoshop, or even The GIMP: a brief tutorial with Microsoft Paint and a link for Mac are below.  Basically, you just find a good background and paste a cutout strategically on top.  You can find high-quality cutouts I made here (holding hands manip) and here (original via x).  Feel free to start a new post for your edit, but please tag me if you use these cutouts.

Link for Mac

MS Paint tutorial:

1) Find and save a background image.  It can be in any format (png, jpg, etc).  Remember to check usage rights and credit the photographer accordingly.  The background should be shot from approximately the same perspective as the cutout: in this case, eye-level and more-or-less straight on.

2) Save the cutout.  It will have a file extension of .png.  Don’t change this.

3) Open the background image in Paint.

4) Change the “Select” setting to “Transparent selection”:

5) Click “Paste v” and choose “Paste from”:

6) Locate the cutout image file and click “Open”.

7) Woah!  They’re giants!  You might want to zoom out for this.  Click “Resize” and experiment with smaller percentages.  Or, you can save time by measuring it.  The percentage you need will depend on the size and perspective of the background image.

I used ~30% for this edit:

What a beautiful location!  I was really inspired by it, so, bonus: