You basically clean your wheel arch very very clean with soap and water, remove grime, tar and whatever. Then you tape up your spension and then just spray the rubber coat on the arch. It 'deadens' any noise that comes from there.

No need to carwash unless you planning on doing it there (cuz the drive home will dirty your arch again).

Wash the wheel well at home with a brush and car wash. Let it dry. Use newspaper and wrap up your suspension and any part that you don't want to spray, then just spray the arch itself (not the mudguard leaf, that one is removable. Spray the car body/chassis portions only). Just only need to make sure you don't spray anything that can move or is removable.

btw, the drying process depends on how thick the layer you spray. Usually 1-2 hours is more than enough

I'll be doing mine this weekend. Will keep everyone posted.

zgerm : it's worth noting that my car is a Wira which is generally noisy to begin with. But 3~4dB of sound reduction is not bad considering I didn't need to pay much for this (compared to sound proofing) and it's easily applied.

Also, if you DIY, make sure you cover your floor nicely. I have rubber stains now on the my floor which I can't remove. Sigh.