You'll need two spanners, I can't remember the exact sizes. One is to loosen the lock nut, while the other adjusts the height (and hence the tapet clearance). If you're adjusting while hot, work quickly, otherwise wait overnight and adjust to cold clearances.
Loosen the lock nut, stick your gauge in and adjust until you're happy I find the clearance shifts slightly when tightening the lock nut, so keep this in mind.
Also, if you're rocker cover gasket is a bit second-hand, maybe replace it at the same time.
Hope this helps.