Biasing to 50% is simply put, just using the trimmers to get half the voltage at the Drain leg of the JFet transistors, compared to the input voltage.
Meaning that if your main voltage is +9v, then the trimmer is adjusted so that you get +4,5v going to the transistor. If the main voltage is +18v (like in this case) then you set the trimmer to +9v
In order to measure this, you use your multimeter, and measure between ground and the Drain of the JFet transistor (Drain is the top leg of the transistors in this case, except for Q2 that is mounted uside down/fliped 180 degrees)
JFet transistors often vary alot in quality/specs (a bit like Germanium transistors), so half voltage/50% is not allways the perfect value/setting, but a good place to start. From there on I allways adjust by ear bringing the voltage up and down around this startingpoint to find the sweetspot, something that can take a bit of time if there ar many transistors to bias/adjust, as you'll need to go a bit back and forth before you get them all as you want them.
Remember to use sockets for these transistors, as you might stumble upon some that are so off spec that you never get them to bias properly. Specially with the J201's which are starting to become difficult to find with good specs.
Just like with Germanium transistors, you allways buy a bunch of them.
If you want to learn the basic technicalitys of the Fet transistors, then this is a quite nice tutorial: http://www.electronics-tutorials.ws/transistor/tran_5.html