Here is where it gets tough.  You must try many methods before you are satisfied with the one you like the best.  This is where you achieve the form and the values and light but you also must protect the likeness.  The more you paint, the more your brain will understand which strokes of the brush and which color mixes will produce the form and texture you need.  Practice looking.  See light and value and texture.  Decide which brushes work best for different effects. Read every article, look at every web site, study masterpieces in museums and galleries.  If you are fortunate enough to have access to a really good teacher, you may gain insight in a more straight forward way.  Don’t give up.