In this case the stamps have been cut to shape and the margins have been added. Then fake roulettes have been added.
Key points are that the added margins are much whiter than the internal white areas of the stamps. Notice the difference between the margin and the area above the back of the swan on the 6d. Notice the difference between the margins and the lettering on the 1/-.
Where the design joins the margin, the design has been painted in a different colour.
If the scan was of a higher resolution (1200dpi) these details would leap out.
The first rule of expertising is MAGNIFY. Many details will appear that are not readily apparent will stand out when the image is larger.