Drain all anti-freeze from the radiator.
Install a Flush & Fill kit and follow the instructions for flushing the system completely.
Any residual coolant or antifreeze left in the system will cause the sealer not to work. Make sure to flush it really well.
Add water to the cooled engine, but leave enough room for the additive.
Add KW Permanent Metallic Engine Block Sealer.
Top up the engine with water, then bleed the air.
Continue topping up until full.
Replace radiator cap.
Idle car for 1/2 hour, or whatever is recommended on the label of the product. (Most "decent" products require you to leave it in your engine for several driven miles...)
Drain all sealer/water, leave all the orifices open and allow car to dry for 24 hours.
Flush once again with water.
Drain water and fill with recommended coolant/water mixture.
Remember to bleed the air from the system by opening any bleeder valves and removing the radiator cap and running the engine until warm, while topping up with water until it can't hold anymore.
This is a method that has worked for some people when faced with a gasket problem.
Whether it will work depends on the location and type of leak (head gasket vs cracked head) and its severity. Some products are specified for a certain type of leak.
A proper head gasket replacement is usually a better fix, but can cost more than the car is worth. (In some rare circumstances, the stop leak fluids can actually work better than a head gasket replacement. In others, they don't work at all
