Find a simple still life subject with highlights and very dark darks. Something shiny would be good, like a tin can with the label removed, or a cut glass vessel. If the reflections seem dauntingly complex, charge ahead anyway. Work in monochrome.
The idea is to simplify by eliminating subtlety in your first version. Try using a middle value wash for the whole shape, reserving only the very lightest lights. Add the darkest darks when the first layer is dry, then assess where more subtlety is needed.
Make another version including all the information you want.
Now put the object and the 2 studies away and make one more monochrome version from memory.