You kind of sculpt out the outlines. Start with the two circles as the base forms, and add smaller and smaller forms on top. Your drawing will not necessarily be an exact replica of the photo, but it should look like an owl.
For example, after adding the sphere for the head and body, you can then add a triangle for the wings, some cylinders for the feet, and some triangles for the ears. Next you can carve out the eye sockets and lay eyes in them. Next you can break up the large wing shape into smaller and smaller ellipses until you have convincing feather shapes. Next you can add a pyramid for the beak, and then carve holes into that form for the nostrils. On the sphere of the eyeballs, you can then carve circles into it for the pupil.
You just analyze and break down the primary, secondary, and tertiary shapes. It's difficult at first, and you won't be completely successful for a long time. But that's what getting better at drawing means, its being able to draw compounding forms convincingly on top of one another.