Has anyone done this, im looking to change the Ones on the bonnet any tips, what's the required measurements, gap between letters? Thanks

Easy peasy if you buy genuine items. They come split as Range and Rover and the decals are already spaced correctly in their application cardboard tape. If you don't buy genuine then there's no guarantee the spacing would be as it should.

To install do one side at a time, then you can line up the new decals to the old. Remove them all at the same time and you'd have nothing as a reference.

Getting the baked on glue residue off was a nightmare, I tried everything short of a blow lamp.

Be prepared for a battle!.

I had the bonnet resprayed and when I first got it I liked the look of it without the letters, now I want to fit the black letters. Having a look at what iv bought it looks like it comes with a template, I'll give it a go.

If you have the template, you'll be fine.
Set it up and tape it inthe correct place, then stick your letters on  ON THE WAY: BMW F21 M140i
MERCEDES GL350 - Metallic Navy Blue w/Grey interior

GOING: BMW F31 335D MSport+ xDrive Touring
GONE: Land Rover Discovery 4 HSE - Stornaway Grey w/Black Interior (wish we'd kept it )
GONE: BMW F10 520D MSport - Carbon Black
GONE:MY12 RRS SDV6 HSE Sumatra/Ivory/Grand Black Privacy glass

If you want them straight, which i'm sure you do, then whatever you do, don't ask awd to give you a hand.