You use a spring compressor. It'* a piece of flatbar that bolts to a rocker hole and is used to then press down with fork over the spring to compress it. Then you remove the two split keys around the now protruding valve stem, release the lever and the spring is free. Very simple.
The real trick is holding the valve from dropping into the cylinder....very bad news.
Three methods, I know of.
1. Remove head and use the traditional method
2. Use compressed air to pressurize the cylinder so the valve cannot drop
3. Stuff a rope inside the spark plug hole to jam the valve between it and the top of the piston.