I would recommend rapid tac over soapy water, it's designed specifically for putting on decals and you don't have to worry about accidentally using to much soap. I put plenty of decals on dry but I don't recommend it to a beginner. I recommend sanding any nibs of dirt before you apply the decal, these chrome backed decals won't lay flat over dirt and it will be very noticeable.
https://www.amazon.com/RAPID-Applica.../dp/B006RMC8J6