Squint and step back.
Two of the most important words I can tell a painter, well three. Squint and step back 1. Squint 2. Step Back By squinting you will be simplifying leaving out details , see edges, hard and soft.
You can then observe the large shapes and masses that you'll want to translate to your canvas. You will create the essence of your subject, the form in light in shadow. You will see the weak or strong areas, what you might change when you put the subject on canvas.
To paint a strong beginning, it is important to simplify. Squinting is the most important tool a painter can use. The second most important thing that you can do is to step back from your canvas and view it from a distance.
Step back several times as you paint to compare and see the relationship of the painting to the subject. Or, if your space is too small, use a mirror to give you the extra distance. Viewing in a mirror or placing a painting upside down can give you a fresh view to see what your next step will be.
There is a time for a painter not to squint. Don't squint at your painting or when you are viewing color. Keep your eyes wide open and relaxed. Know what your painting looks like. If you have trouble remembering to squint or to step back, write a note on your easel.