1) Go to jetphotos.net, airliners.net, planepictures.net etc
2) Pick a mostly white plane
3) Remove any markings still on the plane. Name, logo, the whole shebang. Make the plane completely white. Just use MS-Paint or another photo editor.
4) Save the file.

5) Go to jetphotos.net, airliners.net, planepictures.net etc
6) Pick the livery you want to paste onto the white plane you created earlier. Make sure the plane is at roughly the same angle as the white plane you made.

7) Open a reasonably advanced photoeditor. Paint Shop Pro, Photoshop etc.
8) Open the plane with the livery you want to copy.
9) Copy the part(s) of the livery you need to copy.
10) Paste the livery parts as a new layer into the white plane.
11) If necessary rotate and/or resize.

12) If necessary repeat step 9 to 12. For instance once for the fuselage, once for the tail, once for the winglets.

13) Open the back-up version of the white plane.
14) Copy the windows of the fuselage.
15) Paste as new layer in the modified photo. Because earlier you pasted the livery of another plane over the fuselage, you will likely have lost some windows.

16) Save file.
17) Upload.
18) start gloathing. 😀