One scallop at a time with a tapered round rod, always working from the beveled side only. Match the rod diameter to each serration, take a few light strokes, move to the next, then knock the burr off the flat back with one very light pass on a fine stone.
Serrations are a series of concave scallops. A flat stone can only reach the raised points between them, so working a serrated edge on a bench stone flattens the teeth off and eventually turns it into a bad plain edge.
A tapered ceramic or diamond rod fits inside the scallop, which is the only way to renew the actual cutting surface.
Almost all serrated edges are ground on one side, with the other side flat. Sharpen only the beveled side, matching the existing angle, and leave the flat side alone except for the final burr removal.
Sharpening both sides destroys the geometry and there is no way back.
Find the rod diameter that seats in the scallop. A few light strokes per serration, checking for a burr on the flat side. Work along the whole edge, then lay the flat side on a fine stone and take one very light pass to remove the burr.
See can you sharpen serrations on a flat stone for what happens if you try.