Each letter represented a "length" of road: Y= Yellow, G =Green, O = Orange, R = Red, B = Blue.
So, travelling in south-westerly direction from the start to the first X-roads (= 1st Y), turn R.
Go to next junction of yellows and turn L (this stretch is 2nd Y).
Follow to T-junction and turn R (3rd Y).
Ignore next Y on R (stretch to this point is 4th Y).
Go over railway, round to the R and take next L (this was the 5Th Y).
Travel to roundabout (final, 6th Y).
Turning L at RAB then take next L to travel in a SW direction down the A71.
The first tiny bit of road between your arrival at the RAB and your exit from the RAB is your 1st stretch of G road.
The second stretch is the bit down the A71 until you turn off to the R onto the Y.
The rest follows on from this as you will see from the route map.
You had to be careful going round RABs which is why we inserted the [N] and [f] instructions to help you along.
You also had to be aware of the boundaries of the map so that you could identify which roads were No-Through-Roads (NTR).