The thickness of the masking paper cannot allow the surface of the second stamping to get close enough to the edge of the first, resulting in a halo effect. Illustration 5 shows what properly masked and overlayed images can look like. Simply stamp your masking image so that part of it is atop the sticky strip, cut it out and you are ready to go. Actually, any old scrap paper will do, provided it is not too thick. They are several temporary adhesives on the market with which you can make your own self sticking masks, but some I've found lift the paper, leaving a rough spot.
Some adhesives leave a partial residue of stickiness, too.
This is usually not a problem unless you are combining masking with embossing. If this is the case, the embossing powder will stick to wherever you have laid a mask. One good way to get around this is by using a masking image without any adhesive. Since you normally put the secondary image either to the right or left, or to the top or bottom, it is possible to make a tab on the mask to help in holding it in place. Simply draw and cut out a tab attached to your masking image on whichever edge will not be included in the secondary stamping.