I've built and used this one. It works great - all you need is a voltmeter (DVM) to read the voltages as indicated. Note that Idss is inferred from a voltage measurement.
What you want to do is to make a log (or use a spreadsheet) with two columns of numbers: Vp (V) anmd Idss (mA). Take your set of set of JFETs and label each with a piece of whice masking tape. You may choose simple letters: a, b, c, d... Measure each one and record the measurements along with the label ID on your log. Now when you go to use them you can note the Vp and Idss readings for the particular JFET.
Another approach is to rough sort the JFETs according to Vp, Idss measurements. Usually, if the Vp for a two JFETs is close, so is Idss, so you can sort all JFETs within a certain range of Vp and put them in a bag together, labeling the bag accordingly.
Note that if Idss or Vp are way off from the spec for the JFET, it's a bad part and you should dispose of it (lest you use it by mistake later).
By measuring my JFETs I have been able to avoid trimmer pots for builds like the Wampler Plexidrive. Mainly, it's just good to know what you have so that there will be no surprises.