The selector valve is a owners modification. The factory just put 2 T fittings on the valve and connected both the midlift and 3 point hoses to the T's. So you are moving both at the same time. With the selector valve you remove one of the T fittings and replace it with a hose going to where you want the selector valve mounted. Then you need two new hoses to reach from the selector valve to the two cylinders. There are a number of threads here about installing the selector valve, and there are 3 options for the velve. Some have used a electric valve, others have used a push/pull valve and others have used a 90 degree rotary valve these are listed in descending order of $$$ spent.
This member has 2 kit options for tractor type and desired valve locations. These can save a lot of time, measuring and guess work.
If you need to do this, it will of course be on the backburner for a while