run a thick bead of bond along where it touches your bumper and then around 5mm from the edge run a thin bead along the rim of the diffuser.. fit in to place and attach clamps so that it holds it tight against the bumper... once its dried and set run a thin bead right around the lip of the diffuser so theres no little gaps or anything and looks flush... have white spirit handy in case it smudges on to the bumper... have fun =)