Step 5: Operation
Put your wash in the vat, close it, and gently heat it (over the stove, campfire, whatever - heat is heat). Watch the thermometer rise.
As previously mentioned, ethanol boils at 78oC. When the thermometer reaches this point, and remains steady, it means that the vapours surrounding it, and passing down the condenser is mainly alcohol, with some water.
Catch what drips out of the end of the condenser - that is your distilled spirit.
Keep an eye on the temperature. If it starts to rise above 78oC, the bulk of the water is starting to boil, and the vapours you collect will now be making your spirits weaker. You also run the risk of concentrating fusel alcohols in your sprits.
(Fusel alcohols look slightly oily when they drip. If the drips from your still start to look odd, stop the process and save what you have so far.)
How much can you expect to collect?
If you are starting with an alcoholic content of 5% ABV (as many reasonable bitters are), then you will get only around 5% of the volume you put in the vat. That is, one fluid once per pint of wash.