Compare your rough body to a photograph of a gull in a pose close to the one you want to sculpt. Add extra modelling compound to areas that need to be more rounded, and check the body length for the tail length and position. Add a section for the tail, blending it in to your body.
The final modelling is placing thin layers of modelling compound for wings. When folded, wings have two or three obvious sections, check on a photo to see how they are arranged.
Continue to shape your rough seagull, checking its balance every once in a while to make sure that it is not too heavy on one side, or on the head or tail end. To check it's balance, set it so that it 'sits' on your work surface. It should sit properly without falling forward, backward, or to the side.
If you are using epoxy putty to model your seagull, make sure you leave an indentation where the leg wires will fit. You can insert the leg wires into the gulls body when you have the main belly shape finished, and allow the putty to harden around the base of the wires to hold them. If you are using polymer clay or lightweight air dry clay you can add the legs after the main body has set up (air dry clay) or stiffened (polymer clay).