Check for cracks and missing stones, then brace the weak spots with timber stakes and lash them to the masonry with rope. Fill big gaps with a lime‑sand‑stone mix, let it set, and tamp down loose sections with sandbags. Don’t rush in with cement—old stone wants to breathe, and a quick fix can crack it faster. If a wall feels too thin, add a wooden lintel before you break it open, and always keep the load balanced while you’re setting up the base.