If you are attempting a larger decal install over 10", an installation kit is a must. The professional application fluid in this kit allows you to lay down your decal without it sticking immediately for perfect placement.  This fluid is not a homemade brew such as soapy water.  It is professional grade application fluid.  Soapy water and window cleaners have chemicals in them that can damage vinyl, glue, or leave behind residue. A squeegee allows you to apply decal without worrying about gouging decal or surface.

Professional Installation Kit