Before removing the upper intake plenum, first, reduce fuel pressure and disconnect the battery. Then, remove the air intake duct and engine cover. Label and detach from hoses and electrical connectors connected to the plenum or throttle body. Through undoing the upper plenum bolts in reverse order, remove it together with its throttle body. For placing upper plenum into position after cleaning mounting surfaces and removing old gasket material. The bolts should be tightened accordingly using suggested sequence. Remove lower intake plenum including upper intake manifold, label and disconnect hoses and electrical connectors on them as well as remove plenum support brackets. In addition to that clean mounting surfaces install new gaskets and locate lower plenum on top of intake manifold while tightening bolts in them. Starting with upper intake plenum removing it from engine, labeling and detaching any remaining hoses followed by fuel rail with injectors release then all manifold mounting nuts/bolts should be unscrewed backwards following a gentle use of a scraper that clears away old gasket material. Clean exhaust manifolds before laying new gaskets in position on cylinder heads prior to tightening lock nuts in required order which is prescribed by manufacturer specifications for such engines. Finally, get running motor checked up for leaks firsthand before you finish working on it again without checking your work thoroughly first since you could miss something vital given how many minutes will go unnoticed till completion of whole process unless there's an error or two during installation not caught right away resulting eventual damage beyond repair causing further downtime later down line.