Valve seals

Have heard that valve guides can be replaced by removing spark plug, pressurizing cylinder with shop air, then removing valve spring and keepers, how do you compress the valve spring and keepers? Any help?

I've heard of this method, but I don't recommend it.
Too easy to drop a valve into the cylinder, then you've gotta pull the head off anyway.
If your engine has too much valve/piston clearance then the valve will probably drop down just to far to have the spring and spring seat clipped back on it (if you happen to knock it or lose air pressure).

To answer your original question, compressing the spring will be done with force down from the top, whatever way you find easiest.

seals yes, but not the guides. Well, ok I have done a guide before without removing head.

Have heard that valve guides can be replaced by removing spark plug, pressurizing cylinder with shop air, then removing valve spring and keepers, how do you compress the valve spring and keepers? Any help?

how you compress the spring depends on the engine in question. SBC, there's a tool that will compress the spring from the outside.....

Many engines the springs are recessed in pockets that make it very difficult to do this.

IMO, unless its an engine with known valve guide seal problems, the odds are you need to pull the head and freshen it up if the seals are shot. Replacing only the seals is a bandaid and very well may not make any difference, or will not last very long before the problem resurfaces.

You might be able to put on a set of umbrella seals to keep oil away from worn guides/bad seals.