I've been doing some research on stills, since my spearmint is budding and I was thinking of extracting the essential oils. Mint is great in tea, I think. Generally the peak time for harvesting essential oils from herbs is right before they are in bloom. Also, for mint at least, it's best to pick it in the morning, before the sun comes up. Sunlight will destroy some of the oils.
Making a still (steam distiller) is pretty simple.
To extract the oils you just force steam through the plant material. This steam will pick up the oils from the plant. Then cool the steam off, and it will turn back into liquids. For example route tubing coils through ice water. This separates into essential oils and water.
The tubing system looks something like
[ boiling water ] --> steam --> [ plant material ] --> steam --> [ cold coils ] --> condensed liquid --> [ oil/hydrosol collector] --> open air
It's important that the steam is not lost (holes in tubing, loose connections) because that's what's carrying the oil.
The safest materials for food, might be three sealed (Pyrex) glass containers. One container for water, one for the plant material, and one for the collector. Cork might be the best material for joining the tubing to the containers. Although I'm not sure the best material for the tubing. A lot of the designs I've seen use copper. But also I've heard copper is not ideal, because it can interact chemically with some essential oils. From what I can tell, the best materials would be all glass tubing, and using a blowtorch for shaping.
I happened to run across a video of a couple of Appalachian moonshiners on You-Tube. It's a similar process of steam distillation, provided they don't blown themselves up. :) (alcohol evaporates at a lower temp than water)