Clean the crack and remove loose material
Widen hairline cracks slightly with a utility knife if needed
Fill small cracks with spackle or wall filler
Fill larger cracks with joint compound or patching plaster
Apply fiberglass mesh tape over recurring cracks
Let the filler dry completely
Sand the repaired area smooth
Apply a second coat if needed
Prime the patched area
Paint to match the wall
Check for structural movement if cracks keep returning
Call a professional if cracks are wide, diagonal, or growing