I've only ever found vague instructions, e.g. failing to mention how much chili (dry or fresh?) should be extracted in how much ethanol. How about a soxhlet extraction? Then dilute the extract with sugar water till it's no longer spicy. I think it's crucial to let the panel of tasters start with the highest dilution instead of burning their mouths at first and then expect them to tell if a high dilution is a little hot.
The problem is it doesn't work exactly at all - it's a whole lot of guesswork & subjective impression. It also gets less accurate at higher reaches.
A weakness of the Scoville organoleptic test is its imprecision due to human subjectivity, depending on the taster's palate and number of mouth heat receptors, which vary widely among subjects. Another shortcoming is sensory fatigue; the palate is quickly desensitized to capsaicinoids after tasting a few samples within a short time period. Results vary widely (up to ± 50%) between laboratories.
That's why these days it's done far more scientifically, using high-performance liquid chromatography (HPLC)
Since the 1980s, spice heat has been assessed quantitatively by high-performance liquid chromatography (HPLC), which measures the concentration of heat-producing capsaicinoids, typically with capsaicin content as the main measure. As stated in one review "the most reliable, rapid, and efficient method to identify and quantify capsaicinoids is HPLC; the results of which can be converted to Scoville heat units by multiplying the parts-per-million by 16.
The Scoville scale these days is just a 'comfortable' scale for chilli buyers. Note any commercial pack that displays it will give a broad range of potential, sometimes just as 1 - 5 chilli icons.