Take pictures on a flat surface like the floor or front deck.

Lay a ruler or tape measure on the surface and make sure to get 30" of the ruler in the picture. If you're too far away from the ruler and get 40" in the picture you will lose detail.  Be sure to get as close to 30" as possible.

Put the ruler on the edge of the picture, not in the center.  That will cause problems when recreating the camo.

Take the picture looking straight down.  Not at an angle.  If you take the picture at an angle it will distort the camo.

Try get get the lighting as good as possible.  Shadows and glares will cause issues when recreating the camo.  Outdoor lighting is best.