I use about 2% solution straight into the water tank. Run 1/3 through the system including the grouphead, wait a while, run another 1/3 through, wait a while, run the last through then follow with clean water until you can't taste the acid. Job done.
Also works for the steamer wand but since I never pollute coffee with cow sweat* I don't do this often.
When the portafilter is really filthy I just clean it with caustic. Warning: only do this with machines that have nickel plated brass portafilters, not the cheap ****** aluminium ones. Caustic eats aluminium.
* Mammary glands are modified sweat glands, so milk is basically cow sweat with extra nutrients. Think about that next time you put it in your coffee.