Do it slowly, if you are new, be prepared for several attempts
Best advice I can give is this:
+ Let the
GS cure slightly before you sculpt it, this will make it harder to deform, and less sticky, meaning it can be easier to work with into flat shapes
+ Keep your tools wet, or give them a very thin coating of vaseline (or something similar).
+ Put a small triangle of relatively flattened
GS (about 2/3 the size of your intended eagle size) down on the backpack. You will eventually squish this into full size.
+ Use a sharp pointed tool to "empty" out the space between the wings and the head, pushing the excess
GS towards the wings and thus forming the basic shape of central double heads with wings enfolding them.
+ Using a flat edged tool, press gently at a very slight angle in sections along the "wing" to form the stepped radiating folds seen commonly in many eagle designs, moving from the top wing edge down to the middle of the body/wing.
+ Once you have the wings done, you can then concentrate on forming the faces (these will be the first things you wreck if you do the wings last

). By again using a quite sharp tool, you can push and pull the
GS into shape, as well as drill/press the
GS to form eyes, feathers, etc depending on the size of the design and your own skill level.
Not sure how much of that makes sense, but that is how I would attempt to do it.