Forget all you know about mixing cement mortar, because it's not relevant to lime mortar. That's that out of the way.
Put your gloves, dust mask and glasses on, as you don't want to get it in your eye or breathe the NHL in.
NHL 3.5 as already specified and mix it 2 parts sharp sand to 1 part lime. Gauge the lime and sand using a bucket and don't tamp it down, as you'll use loads of lime. Don't use building sand or yellow soft sand as it's too fine. you want a bit of grit.
Put it in a belle mixer, if you have one and mix the dry ingredients for a while. Then add some water, you want enough water for it to start balling up and no more.
Then add a couple of rocks the size of skittle balls (heafty, but not massive), granite works well because it doesn't crush up. Lime mortar likes to be chopped and crushed, which is what the stones do.
Keep mixing it and add a little water, not too much as it's easy to make it too wet and runny. Stiff and sticky is what you want and it should stick to a trowel even when it's upside down.
You'll have to put your colour in at the beginning, but I don't have any experience with that, so I'm not going to comment on it.
The lime mortar wants to be sticky enough so that it hangs together in the mixer and the mixer drum is not caked with clumps of dry mix at the bottom. If it's too wet, youll have to add more lime, but this will screw the colour up and it won't be consistent between mixes. So gauge everything.
Once it's mixed, dump it in a barrow and get to work.
Lime mortar has two sets, an initial set where it goes stiff quite quickly and a hydraulic set when it starts to absorb CO2 and turn into calcium carbonate. The initial set can be broken by smashing the mix with a lump hammer. Once you've done that it becomes plastic again and takes a while to set, as it's relying on the hydraulic set this time and that can take a month to harden or longer if it's cold.
Don't add cement, ever, as that turns it into a cement mortar and defeats the object.
Using a finger trowel and a plasterers hawk or tyrolean plate, point your wall and overfill the joints. Leave it proud and wait overnight before finishing the joint, as you'll end up with white or black bricks if you do it earlier. If your pointing goes over a few days, cover the mix in the barrow with some plastic and knock it up again in the morning. Don't add water unless it starts to go a bit dry and won't stick together.
Protect from the sun by covering it with hessian and misting down if it's hot and don't point when it's frosty.
That's about it. It's not rocket science although it can get sciency if you want it too and you can also make it overcomplicated if you feel like it, too.
