Before you start doing any active exercises, the first thing to do is ensure your pup sits in the correct way. Looking from above they should be completely symmetrical, and both back feet should be parallel with the body, nothing should be at an angle or sticking out from the body
When in the sitting position use a treat to encourage your pup to move into a standing position. When they are standing upright with their back parallel to the ground then get them to sit back down. Do this 5 times, have a short break and repeat 2 more rounds of 5. By doing this exercise 1-2 x daily, you will build and strengthen the muscles in the back legs (a bit like humans doing squats) and this will enable your dog to stand in a normal position with a bent knee and bent ankle (see photo).
Dogs with poorly developed muscles either straighten their knees and ankles to effectively lock them in position (leading to problems like slipping kneecaps, cruciate problems or ankle arthritis) or they are unable to hold their posture and walk with a very bent knee swaying posture (frequently seen in German Shepherd dogs)
Correct Sitting Position
Sit to Stand