Changing valve springs on car

The easiest way is a surplied air source to keep pressure on the cyl. via spark plug hole. And a on head valve spring compresor.
Comp cams sells a tool that bolts on the head like the shafts and has a handle on a pivot that pushs down on the spring.
This is a $90 tool. Expensive by compare to what others have showed you. I myself can use my body weight to compress the spring instead of turning the handle on top of the KD tool. with my aching hands. Since I have an air compressor, the other expense would be the air fitting.