How do I replace a faulty head gasket?

Removing the cylinder heads on boxer engines while the engine is still in the car is possible but not practical. To get the best result, remove the engine from the engine bay and mount it on an engine stand. Then remove the cylinder heads and old gaskets. Clean the surfaces properly and then mount new gaskets, following the steps listed in a shop manual. Torquing the head bolts in the correct order and to the correct torque is critical. Then mount the engine back in the engine bay. It's a big job and not well suited for a DIYer who's never done this before