GM valve stem seals are pretty much all alike. They can all get hard and brittle over the years and leak oil into the chamber...giving the little puff of blue on startup.
Easiest to do with the heads off the car, but can be done with the heads on.
Those are the valve seals. (green-blue rings around the the valve stems)
As willwren said, you'll need new rocker arm bolts and a torque wrench to tighten the rocker arms back down.
Other stuff: Valve Spring compressor (little type that fit over the springs and doesn't hold the valve..looks like a little gear puller) Small flat blade screwdiver, like a jewlers screwdriver, small rubber mallet, and a magnet. Compressor & compression guage set is really handy. (You don't need the guage, but the hose fitting that screws into the sparkplug hole usually fits a quick-disconnect air compressor hose too..allowing you to use the compressor to pump air into the cylinder through the spark plug hole.)
Bring each cylinder up to TDC firing. The rocker arms should be loose enough to wiggle them at this point, since the lifters are on the cam'* base circle. If you have a compressor, hook it up in place of the spark plug and presurize the cylinder to hold the valve against the seat. If you don't have the means to put air into the cylinder, you are not out of luck. Simply MAKE SURE the piston is at top dead center, the valve cannot drop all the way into the cylinder with the piston at the top. Exercise a bit of patience and use a magnet to lift up on the valve stem while you are putting the locks back in.
Remove the rocker are and use the spring compressor to remove the valve spring. Hint, when the spring is fully compressed, use the tiny screwdriver and magnet to remove the two locks that hold the valve in the hat of the valve spring. Years of cooked oil can make them stick in place, hence the screwdriver and magnet. You may also need to wiggle the compressed valve spring around to make enough room for one of the locks to come out...once one is out, the other stem lock will come right out.
Be carefull. Don't drop any parts down into the motor, or you WILL be taking the motor apart to retrieve it. (Pack a rag into the opening where the pushrods come up through as a little insurance.)
Remove the old seal, clean and lube the valve stem, and install the new seal. A deep socket is handy to use as a press to push the new seal down over the boss in the head.
Reinstall the spring, locks and remove the spring compressor. Lightly tap the end of the valve with a rubber mallet to make sure the locks are seated. Reinstall the pushrod and rocker with a new bolt. Then move on to the next valve.
About two hours later you will have all six done and can reinstall the valve covers.
By the way, since you have the rockers off, soak the pushrods in some solvent and blow out the tubes to make sure there'* nothing restricting oil flow up to the rockers. Check that they are straight by rolling them on a piece of glass. If the pushrods pass muster, put them back in. If not, replace them with new ones.