[Scale-Master]

Make sure you are using good sharp scissors. Cut the image a little larger than you need it to be. Use decal solvent to help make it conform, then trim the excess with a sharp knife.
Never put tape on the face of a decal unless you are trying to remove it and discard it. You can use tape on the back side (paper) of the sheet, but be careful and remember to reverse your design.
Decal solvent really doesn't help the adhesion, just the pliability.
Applying clear over it after it is applied is the preferred method, but in some cases it may need to be sealed with a thin coat of clear before use.
Test to make sure the clear is compatible with the decal before spraying the whole sheet. A thin coat is all you need, and decal solvent should still work with it, just a little slower... - Mark