I didnt bother with soap.
I took the lower grill off, measured it out, pushed the vinyl down onto the lip (with tha backing on at this stage) and creased out to the edges, cut it out ( just a little bigger than the creases) and stuck the vinyl down. Used a credit card to smooth it out. Then hit it with a hairdryer.
Was it easy getting the centre grill out, sorry haven't taken it out before, that would seem the easiest way to apply the vinyl.