Most moss projects start with collecting a little, and done thoughtfully that is fine. Done carelessly it scars a place that took years to grow. A few simple principles keep you on the right side.
Little and often, from many places
Take small pieces from several spots rather than lifting one patch whole. Moss regrows from fragments and from the edges of what is left, so a light, scattered take recovers quickly, whereas a bare scrape can stay bare for years. Your own garden, walls, paths and pots are the easiest and most guilt-free source, and they usually hold more than you think once you start looking at the shaded, north-facing and permanently damp corners.
How moss recovers, and why that guides you
Moss has two ways back. Almost any living fragment left behind can grow on, sprouting new shoots and re-anchoring itself, and the spores shed by mature plants settle and start fresh colonies of their own. The threads that hold moss down, the rhizoids, are anchors rather than feeding roots, so a torn cushion is not maimed the way an uprooted flowering plant is; it simply needs to grip again and carry on. This is why leaving plenty behind matters so much. It is not mere politeness, it is the actual mechanism of recovery. Spreading, mat-forming mosses knit back over a gap fairly quickly, while the domed cushion mosses rebuild slowly, so treat a cushion as something to sample lightly rather than harvest.
Where you may, and may not
On your own land, collect freely. On other land you need the owner's permission, and in many places removing plants from the wild without it is not allowed. Nature reserves, protected sites and designated areas are off limits, and some rare mosses and their habitats are specifically protected by law, so a casual handful can be a serious matter on the wrong ground. Town pavements, old walls and waste ground are far less sensitive than ancient woodland, heath or bog, where the moss is often part of a slow, irreplaceable community. If in any doubt about a wild spot, leave it and grow on from garden stock instead.
Do no lasting harm
Avoid stripping banks, boulders and tree bases that hold a place together and shelter other life; that moss is habitat as much as scenery, home to the small creatures covered in moss and wildlife. Take only what you will actually use, and leave the spot looking as though you were never there. The wild does not owe you a moss lawn; a modest start, grown on with patience, gives you far more moss in the end than a greedy first haul, as the growing guide explains.
Cleaning and settling it in
Whatever you bring home, go over it before it joins a project. Shake and pick out soil, leaf litter, slugs, snails and their eggs, and give it a rinse in rainwater, since a closed jar or a shaded wall will happily incubate whatever rides in with the moss. Carry it in a tub or bag rather than a pocket, and keep the pieces damp and shaded from the moment you lift them, as moss dries fast once detached from its patch. Press each piece into firm contact with its new surface so the rhizoids can take hold again, and mist it well for the first few weeks while it settles. Collected gently and grown on with care, a handful from your own walls will spread into far more than you ever took from anywhere.