If you bought a Dimma grill from new, you should have been provided with some mesh. If not, you can pick up a sheet in most colours at Halfrauds.
* Unscrew existing grill.
* Cut the mesh a little bigger than the dimma grill.
* Place mesh behind Dimma grill and use screws to hold it in place.
* Fix to existing grill recess (Not all holes are used).
That's about it to be honest. You may notice that the grill sits pride of the recess a little. This is because the mesh pushes it out a little (approx. 3mm). If you tighten the screws, you can try and get it as flush fit as possible.
Remember to leave a hole for you to release the bonnet release catch.