Removing the valves is simple in principle but a bit fiddly in practice. With the small Briggs engines the valve retainers are normally of the keyhole type, thank heavens, and the valve springs are very weak (small valves and low maximum speed makes this possible). Essentially you need to push the retainer up while holding the valve from rising, thus compressing the spring, which leaves the valve stem projecting through the retainer. You can then push the retainer sideways across the keyhole, to the larger, off-center hole. Once you do that, stop compressing the spring, and the retainer will be pushed off the stem by the spring. You sometimes need to hold the valve down while pushing the spring up. The cylinder head needs to be off, to give you access to the valve head so you can hold it down.
Putting it back together is more fiddly than taking it apart, because you have to push the off-center hole over the valve stem, then push it sideways until the stem aligns with the smaller hole, despite the whole area being full of fingers so you can't see what you are doing. Joe Carroll has found the answer to this. You use two ordinary cable ties to hold the valve spring compressed while you install the spring and then put the retainer onto the valve stem. At that point you cut the two cable ties, freeing the spring.
A valve spring compressor is not needed for mower engines because the springs are so weak they can easily be compressed with two fingers and a thumb. If it weren't for that piece of good luck you'd have to use a special side-valve spring compressor, a simple example of which can be made out of a small piece of 16 gauge plate with a hacksaw and a drill.
