Try tightening the bolts that hold the valve covers down. With age, the gaskets can shrink. Be GENTLE and don't overtighten the screws. They are small screws and might break if you overdo it. Also, if you overdo it, you can distort the valve covers and make the problem worse.
If that doesn't work, you'll be needing to replace the gaskets, which involves removing the valve covers, cleaning the sealing surfaces, installing new gaskets and putting it all back together. See if you can find a Chilton's or Haynes manual or similar. Read through the relevant chapter and then decide whether you and your toolbox are up to the job.