See below

Choose a wide, shallow earthenware pan. Strain very fresh milk into
this & leave to stand, overnight if summertime or for twenty-four
hours in cold weather. Then slowly, & without simmering, raise the
temp. of the milk over a low heat until a solid ring starts to form
around the edge. Without shaking the pan, very carefully remove it
from the heat & leave overnight, or a little longer, in a cool place.
The thick crust of cream can then be skimmed off the surface with a
large spoon or a fish-slice.