So, this applies to all realism artwork in any medium you choose.  I did learn it for painting first, then had to learn how to use a pencil and charcoal after that because I didn't really have any skills with dry materials, but whether you are painting or drawing realistically, it is of utmost importance to remember the following how to tip:

2. Focus on shapes and values, not what it is

That might be a little confusing to you, I say this because even though I paint cows and cattle as my main artistic focus, I am often asked if I can paint a (fill in the blank) too.  The fact of the matter is, since everything I see is shapes and values, I can paint about any subject that I have been given reference for.  I can't make up details that are not there, but I can render whatever the subject is into shapes and values.

Monochrome Hereford Calf       8 x 10 (this was a finger painting with burnt umber, the brush was leaving brush marks so

I used my fingers and a rag to wipe off the lights)

Shapes is self explanatory.  Values means light to dark shades.  I would start out working in monochrome, all one color, and put your reference in greyscale not color.  A photo will often blow out the lights or make shadows appear black,  which they rarely are, so when using a photo as reference this has to be accounted for.  This is one reason a lot of artists like to paint from a real scene, they can actually see what is there instead of what a camera sees first, then translating it. 

One of very first oil experiences involved one tube of Ivory Black student grade winton oil paint and a canvas panel.  I painted the entire canvas black, then using a rag wiped out the light areas using more pressure for the lightest, and less for the medium tones.  I highly recommend this!  It was from a reference photo of Chief Joseph of the Nez Pearce tribe and is shown below:

9 x 12.  33 years ago or so, my first portrait in oil.  It was so much fun to do!