Grout is porous, so dirt and soap scum sink into it. Scrubbing with a brush takes ages and rarely gets it all.
Fill with tap water
No cleaning products needed. Fill the tank, let it heat up and fit the small round brush.
Work one line at a time
Hold the brush on the grout line and move slowly. Pressurised steam lifts the dirt out of the surface.
Wipe as you go
Follow behind with a microfibre cloth to pick up what the steam loosens.
Let it dry
Open a window and leave it to dry. Do a quick pass once a week and it stays clean.