I installed a full vinyl decal set on a pinball before and I primed the sides, sanded to a smooth touch (but still painted, to prevent liquid from quickly absorbing) and then applied a soaking coverage of Windex to the area for the decal. The windex allows for shifting of the decal for a minute or two. Once the decal was where I wanted it, I placed a clear piece of painters tarp down (to protect the decal) and I roller squigeed it down. This squeezed out a lot of the excess Windex and ultimately made for a firm seal. Once one side was on, I walked away for a day to let it dry good. The next day you trim excess off with a brand new razor blade and move to the next decal.
What are you using on top of your CP to protect the decal? I use an epoxy resin. You have seen it before, the heavy clear plastic looking wood bar tops. Spreads evenly to a glossy glass finish, super strong, non-yellowing, wipes down easily. That I let sit for a week once applied.