Crit

(look for elements and traits that repeatedly appear)

The painting is unified by a very limited palette of color — Goya uses a traditional palette of browns and yellows, as well as black and some red.  Color is rather subdued — value rather than chromatic color drives the composition. (tonal-based design)
The image is predominantly dark — the well-established, pervasive low key helps unifies the whole. The lighter areas are rather skarce, and so will provide handy focal areas.

(look for alignments, structures or groupings that organize parts into larger entities (gestalt))
There are a couple of large shapes that group many smaller features — the across the center of the painting figures are clustered into a crowded mass. The prisoners are grouped into a disorganized, irregular pattern, the soldiers on the right, in the firing squad, are aligned into an orderly whole — they are a unit, not individuals, per se.

(look for contrast of any and every kind. Look especially for similar forms that are varied in some way. Look for anomalies — patterns or norms that are broken.)

Lighter areas constrast with the dark.
A few chromatic colors (yellow near the light and red blood at the feet of executed prisoners) stand out.

The main focal area is the man standing, arms raised in an almost crucifix-like gesture — defying his executioners, unwilling to cower before them. His brillian white shirt, the X-like stance, and his emotional expression all serve to draw out attention to him.
Further, the cone of light projecting from the lamp aims directly at him. The parallel lines of the executioner's rifles aim directly at him.
The light itself is prominent — an intense light color among the prevailing darkness. The light suggests that what was done in the secret of night, will be known.

Many areas are quite dark and have little contrasting light accents and very little discernable form — particularly the upper and lower third of the composition.