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 have more than you think.
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; some rare mosses and their habitats are specifically protected by law, and a casual handful can be a serious matter on the wrong ground. Pavements, old walls and waste ground in towns are far less sensitive than ancient woodland or bog.
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 well as scenery. Take what you will actually use, clean off soil and creatures before bringing it home, and leave the spot looking as though you were never there. Treated that way, collecting moss is a gentle thing. The wild does not owe you a moss lawn; grow most of it on from a modest start, as in the growing guide.