Don't.. You will cause more issues than you want to deal with. I recently bought a Freelander with a head gasket problem.. The previous owner had used 3 or 4 doses of this type of product..
It did not fix the problem and cost me a few extra hours to clean it all out as it coated everything
I cant deny it may work for some, but imho this type of stuff just makes a mess.
Fix the problem, don't just treat the symptoms. All you do is create more problems.
The only proper way to fix a head gasket is to replace it..