To support the oils I use archival artboard laminated with matte gel medium to sanded and cradled 1/4-inch standard hardboard (not tempered or marine, which might have oils or resins that could bleed through eventually), or stretched fine-weave portrait linen painted with gesso and sanded and repainted and sanded several more times until smooth. I just recently tried manufactured cradled clayboard, and like that surface also.
I start with a rough sketch on scrap paper — mostly just to decide what shape to make the board. After preparing the board or canvas I usually spend a couple of days doing a detailed underdrawing. When the pencil or charcoal drawing is right, then I start painting with thin layers of umber or gray and white acrylic, slowly building values and volumes until I've gotten the shapes rounded out and placed where I want them. When I'm pretty happy with that, then I start layering in oil color, sanding with extremely fine sandpaper between layers. Of course I am making little changes the whole way through. I use Liquin for a medium and plain turpentineto thin the oils.
The little details, like hairs, lace, or letters go on last with tiny 6 x 0 and 10 x 0 liner brushes over the many layers of background color. A 10 x 0 spotter or round brush won't be as delicate. The not-so-detailed work I do mostly with soft synthetic #4 and #6 flats. I buy the really cheap ones and change them often.
A painting generally takes a month or two from start to finish, usually working a ten-hour day.