It is a highly technical procedure, which requires specific tools and equipment and therefore should be done by a professional and it's referred to as a valve job which entails servicing of the valves, seats and guides. For home mechanics, one can pull the heads, make initial dirt and grime washes and inspections and then take the head to a dealership service department or to an automotive machine shop for the real work to be done. Performing the inspection enables identification of the condition of the heads and other valvetrain parts as well as the awareness of work required and available new parts via working with an automotive machine shop. Dealer service department or automotive machine shop will recondition/ replace valves and seat, recondition guides, check / replace valves springs, spring retainer, rotators and keeper if necessary, replace valve seal and reassemble valve parts, check the installed spring height to confirm is correct. Also, if the cylinder head gasket surfaces are counterform, they will be machined to attempt flat. When the valve job is done by a professional the heads will look as good as new as far as the outside appearance is concerned. Before fitting them back on the engine, these parts must also be cleaned once more to get rid of any remaining metal chips and abrasive dust that may well still be stuck there, post valve work or in the event of a head milling, employing, if possible, compressed air to blow through all oil galleries and channels.