There are really two ways of doing this.
Proper distillation apparatus..........best home use stuff is coming from Portugal just now, we can find you links, but it's not cheap.
Or, an old fashioned pottery alembic.
The former will give commercial grade essential oil, the latter will give very useable oils and floral waters.
The alembic basically stews the plant material and then it condenses on the conical lid which has a spiral from the tip to a spigot that leads out and into a jug or bowl that catches the condensate. The oil usually separates out and floats, and needs strained like the fat off gravy.
Very good for lavender, mints, lemon balm, some roses, bog myrtle. Not good for iris.....for some weird reason that just smells of sewage
cheers,
Toddy