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Start with the size of the hole, because that decides the entire method. A nail hole, a doorknob dent, and a fist sized break are three different repairs, and the reason people end up with a lumpy wall is that they use the small hole technique on a big hole and keep adding compound until it looks like a scar.
I patch walls a few times a year, mostly after moving a picture or pulling out an anchor that failed. The work itself is quick. The waiting is what people underestimate, and rushing the drying is the one thing that ruins an otherwise fine repair.
Nail holes and small dents
Anything up to the width of a pencil is a two minute job. Press lightweight spackle into the hole with a flexible putty knife, drag the blade across at a low angle to leave it slightly proud, and let it dry. Sand it flush with fine paper wrapped around a small block, wipe the dust, spot prime, and touch up the paint.
Two details make this look better. First, run your finger over the hole before filling to knock down the raised lip that a nail pushes out, because filling over a lip leaves a bump. Second, use the smallest amount of compound you can. A wide smear takes longer to sand and creates a shiny halo when you paint it.
Holes up to a few inches
Once the hole is bigger than the compound can bridge on its own, it needs backing. The easiest option is a self adhesive mesh patch, which is a stiff screen with a sticky back. Clean the wall around the hole, trim any torn paper flush, stick the patch on so it overlaps the opening by an inch on all sides, and press it down hard.
Then comes the part that separates a good patch from a visible one. Apply joint compound in thin coats, each one wider than the last. First coat covers the mesh only. Second coat extends a few inches past it. Third coat extends past that. You are not filling a hole, you are building a very shallow ramp that the eye cannot detect. Let each coat dry completely, and sand lightly between coats.
The width is what matters. A patch that is thick and narrow casts a shadow. A patch that is thin and wide disappears. My first attempt at this was a tight little mound about four inches across, and in the evening with the hall light on it looked like a bruise on the wall.
Bigger holes need real backing
For anything approaching the size of your hand, mesh alone flexes. Cut the damage into a neat square with a drywall saw, cut a piece of drywall the same thickness to fit, and back it with a strip of wood screwed to the inside of the opening so the new piece has something to fasten to. Tape the seams, then do the same wide thin coats.
Before cutting into any wall, know what is behind it. Do not cut blind near an outlet, a switch, a light fixture, or the wall behind a sink, a toilet, or a shower, where wiring and supply lines run. If you are not certain what is in there, that is the point to bring in a professional rather than to find out with a saw.
Matching texture
A smooth patch on a textured wall is as obvious as a shiny patch on a flat one. Look closely at your wall in raking light. Many walls have a light orange peel or a knockdown pattern that you stop noticing after a while.
Orange peel can be reproduced with a spray texture in a can, held well back and applied in short bursts on cardboard first to get the setting right. Knockdown means spraying, waiting a minute, then flattening the peaks lightly with a wide knife. Practice on cardboard until it looks like your wall, because you cannot undo a heavy spray without scraping it off and starting again.
Hand applied textures can often be faked with a damp sponge or a stiff brush stippled into the last thin coat of compound while it is still wet. It takes a few tries. Keep a bucket of water nearby so you can wipe the patch smooth and restart if the attempt looks wrong.
Prime before you paint, without exception
Joint compound and spackle are porous. Paint sinks into them and dries flatter and lighter than the surrounding wall, which produces the classic ghost patch that shows up only in certain light. A quick spot prime with a basic wall primer solves it and takes a few minutes.
Every patch that ever haunted me skipped the primer. None of the primed ones did.
Then paint with the sheen and the applicator that match the wall. If the wall was rolled, roll the patch with a small roller so the surface texture agrees.
Where a patch will not save you
Some wall damage is a symptom. If you find a soft, discolored, or slightly bulging area, especially on a ceiling or on a wall shared with a bathroom or a kitchen, there is moisture involved. Filling it hides the evidence and the problem continues behind the compound. Find the source first, and get a plumber or a roofer involved if the source is not obvious.
Cracks that keep reopening in the same place, particularly diagonal cracks running from the corner of a door or a window, are movement rather than surface damage. You can fill them and they will come back. It is worth having someone look at the structure rather than patching the same line every spring.
Everything else, which is the vast majority of the marks a household leaves on its walls, is a putty knife, a little patience between coats, and twenty minutes spread over two days.
