Sorry only just seen this mate.
No I left the diffuser on, there is a little bit of metal that will need filing down on the inside of the new tips - this will allow you to just push them on to the existing exhaust tips.
I just tapped mine on with a rubber mallet making sure they were far enough on to be secure, whilst making sure they were level with the opposite side. I think some people use jubilee clips to hold them on but I just tapped mine on and they've been on for 2+ years with no problems.
Just make sure they are on central to your diffuser holes so they aren't touching the plastic.