I have not done a self portrait in about 15 years.
I used acrylic paints for this.
I learnt that whatever part of the face you are looking at seems more exagerated as you study it so i was constantly having to tone down the conrast of light and shadow as I progressed. In other words i would be looking under the right eye (the one in shadow ) and see more contrast in the shadow but standing back and looking as a whole i could see it was to bold compared to the whole piece. I thinkit would help if i choose a main focal point eg, right eye or nose or something.
I also noticed that the folds on the forehead had thin lines of light at the top which was interesting and something i had never observed.