Filling cracks and holes in walls
Filling is easy to do adequately and harder to do invisibly. The difference is almost entirely in the preparation and in how many thin coats you are willing to apply.
Decide whether the crack matters
Most cracks are cosmetic: hairline marks where plaster has shrunk, lines following a joint between boards, or the gap that opens above a door frame as a house moves with the seasons.
A few are worth attention before filling. Cracks wider than about five millimetres, cracks that run diagonally across a wall, cracks that pass through brickwork outside as well as plaster inside, and cracks that visibly grow over a few months are all telling you something structural. Filling those hides the evidence without changing the cause.
Open the crack up
This feels wrong and it is the key to a repair that lasts. Run the corner of a scraper along a fine crack to widen it slightly and undercut the edges. Filler pressed into a hairline gap sits on the surface and falls out later. Given something to key into, it stays.
Brush out the dust. Filler does not bond to dust. On old, very dry plaster, dampen the area lightly first so it does not pull the water out of the filler before it sets.
Fill in thin coats
Press the filler in firmly with a flexible knife, working across the crack rather than along it, so it is forced into the gap rather than dragged over it. Leave it standing very slightly proud, as filler shrinks a little as it dries.
Deep holes want two or three thin layers with drying time between them rather than one thick one. A thick fill dries on the outside first and cracks across the middle as the inside catches up.
Sanding and painting
Sand with a fine paper on a flat block, not with fingers behind the paper, which rounds the repair into a dish that catches the light. Run a hand over it with your eyes shut: fingertips find ridges that eyes miss.
Seal the repair with a mist coat before painting. Bare filler absorbs paint differently from the surrounding wall, so skipping this leaves a patch that shows through the finish no matter how many coats go on.