Moss graffiti is a living design: a shape, word or pattern painted onto a wall as a moss slurry, which then, if all goes well, grows into the real thing. Done in the right spot it is one of the more satisfying things you can do with moss, but it is honest to say up front that the success rate is low and the results are slow.
The slurry
Blend a couple of handfuls of clean moss with enough liquid to make a thick paint. The traditional binder is buttermilk or natural yoghurt, which helps it cling and feeds the moss a little; plain water works too. Some people add a spoonful of sugar or a little water-retaining gel to hold moisture at the surface. You want a mix you can apply with a brush without it running straight off the wall.
Applying it
Pick a shaded, damp, porous surface: rough brick, stone or bare concrete in shade takes moss far better than a smooth, sunny, painted wall. Paint your design on with a brush, keep the edges crisp, and then comes the only hard part, which is keeping it damp for weeks on end. Mist it daily while the moss establishes. In a genuinely shaded, humid spot it may green up and fill in over a month or two; in sun it simply dries and fails, however good the slurry.
Where, and whose wall
This is lovely on your own damp north wall, a shaded fence or a back yard. On anyone else's property it is criminal damage however green and gentle it looks, so ask first, and do not do it on listed or historic stonework where even moss can be unwelcome and hard to remove without marking the stone. Your wall, your rules; someone else's wall, their permission. It is worth saying plainly, because the appeal of moss graffiti is partly that it looks harmless, and on the wrong wall it is not treated that way.
Designing the lettering
Moss spreads and softens as it grows, so fine detail blurs. Keep shapes bold and strokes thick, the way a stencil or a chunky display typeface reads, rather than attempting thin script that will smudge into an illegible green smear within weeks. A simple word, a number or a clean graphic holds up far better than anything fiddly. It helps to chalk the outline on the wall first, or cut a card stencil and paint the slurry through it, which keeps the edges crisp while the moss is establishing.
Why it often fails, and how to improve the odds
Be realistic: many attempts come to nothing, and the ones that work take patience. Most disappointments come down to the wall and the watering. A sunny, smooth or painted surface will not hold moss however good the slurry, whereas rough, porous masonry in steady shade gives it a real chance. The other half is moisture: the design needs to stay damp for weeks while the fragments take hold, and a single dry spell of neglect undoes the lot. People who get good results either pick a wall that stays naturally damp or rig a way to keep it misted, and they accept that the design takes a month or two to come through rather than appearing overnight. Treat it as an experiment with a decent chance of failure and you will not be let down.