Shut off the source if possible
Use the appropriate fixture or main water shutoff if you can reach it safely. If the break is associated with electrical hazards or a dangerous area, stay clear and get qualified help.
Trace where the water traveled
Water may move through wall cavities, under flooring or into rooms below the break. Visible staining does not always show the full affected area.
Extract accessible water
Once the source is controlled, removing standing water can reduce ongoing migration and prepare the area for drying.
Dry wet assemblies before closing them back up
Walls, floors and cavities that became wet need time and appropriate drying conditions before damaged finishes are replaced.
Talk through the situation by phone
Describe the source of the water, the affected area and whether the water is still entering the property.