Fletch, wow thanks for the compliment. I believe you might actually live near me. If you want I am happy for you to come and have a look at it and I could explain.

Otherwise, I measured, did some figures (divided the wall by four), the diamonds are xcm across and then 2xcm high. then I worked out where the middle of each diamond was (for the top and the bottom) and how far they needed to be across the wall. Basically I pencilled it all in and then used low tack tape to mask and then just painted it having painted the base colour first.
The thin stripes are actually ribbon that I have used a hot glue gun to stick to the wall.

Easy but the calculations and markings are a bit fiddly. I also had to paint the diamonds alternately (ie first and third then second and fourth) cos the tape gets in the way.

I have sent an email as well with my address is you're interested in having a look.