To paint Ophelia, Millais would have used a small transportable 3-legged easel on which to rest his painting.
He would have sat on a stool and probably attached his umbrella into his shirt to protect him from the varied British weather.
Because his work was so detailed he would have used some very small and fine brushes. He would have painted on to a
canvas with oil paints and used a porcelain palette to mix his paint.