Feet end up smaller than you think, so make the actual foot larger than you want when finished. If you are doing a one piece leg, it may be helpful to think of the leg as heartshaped, with the feet added on at the bottom of the heart. Keep that smooth shape in mind when designing. Now for the arms. As with the legs, two pieces give you many options.
Bent arms can be exceedingly cute.
A one piece, straight arm also works nicely and leaves you one less seam to worry about. In general, the shape of the arm will be something like the shape of the leg down to the paw pad. The pad itself can lead straight out from the arm or flare upward, or any other variation you can think of. Think about the approximate size of your footpad and make the paw pad accordingly. Naturally, heads are the most difficult.