While I am not sure about the many brands of skins I have applied, I know I used DecalGirl for sure. As a professional decal applier.... aka parent of three children
. I have learned a few tricks over many years. First is surface prep. Plastics have a release agent applied in some instances and even if not oils from your finger touches will hamper adhesion.
I use a good windex/glass cleaner at first and wipe down the whole of surfaces for the decals, I then use a tack cloth and a high grade of rubbing alcohol for surface prep. This helps your hands as well and removes the oil so that you don't contaminate the prepped surface when applying the skins/decals.
I then use a blow dryer and warm the surfaces for the decals. The blow dryer is used while applying as well to soften the decal to allow tight turns if you get a bit out of alignment, be careful not to over pull decals when warming them. if so pull decal back up and gently warm the decal and it should shrink back to normal. Once you have all surfaces covered. Heat all surfaces a bit more and press decal down to seat and allow adhesive to bond firmly..once cool you shouldn't have any issues with decal separation. That is how I do it, and I have great results. YMMV according to skill set.
I will add that the Skins made with true 3M vinyl seem to be much better than their China counter parts so buy USA!