Probably with a custom tool. Looks like 4 or 5 metal teeth from a course saw, slightly curved, and mounted on a plastic handle. Mine was made by the PlasPlugs organisation but there are probably others around.
The best tool on the market is a dremill but if you don't own one then i agree with buildersmate as they are very cheap from available any main DIY store
piece of 2 " by 1" about 12" long 2mm masonry nail hammered to project through the end with slight angle hey presto grout scraper renew nail now and then or rub up on stoneto sharpen