Exercise, and making your body healthy is important when feminizing the body because there is exercises you can do that decrease the size of your muscles, removes excess fat, while giving you skin that's more youthful, firm, resilient, and glowy. They also make your whole body more healthy. Your body also processes hormone pills better when doing HRT. Your body's metabolism is also healthier, which is very helpful if you plan on doing any surgeries to feminize your body.
These exercises are called "Cardio Exercises", which is the opposite of "Weight Training". Weight Training is designed to increase the size of your muscles. Just like drinking water, Cardio helps move all the right nutrients around in your body to the right places without making muscles bigger. Good Cardio usually involves stretching first, and then anything that keeps your heart rate up for at least 20 minutes a day. Like running, biking, swimming, etc. It can be different with different people though. It's good to record your progress while doing certain exercises to eventually find what works best for you.
What you eat is also important, because you could accidentally do Weight Training if you workout enough to make your muscles sore, then eat fattening foods, and sleep more than 8 hours a day. Weight training involves tearing muscle tissue so the muscle's healing response activates, and they use what calories are available in the body to grow new muscle tissue. If it's a lot of fat calories, or protein, that's what increases the size of the muscles. Sleep is also the main time body chooses to start healing, and increasing the size of the muscles.
This does not mean to go on a starvation diet. I can't stress that enough. That will harm your body in many ways. It's okay to eat healthy foods that are not high in fat calories.
Cardio also involves tearing the muscle tissue. That is how they get smaller. So it's also a good thing if your muscles are sore. Just avoid eating too many fat calories, and over sleeping.
Once you think your body is properly toned enough to stay that way for a while you can do Weight Training that's just focused on the the butt for a bigger gluteus maximus. Squats are not enough for Weight Training just in the butt area. However it can include squats, and also Google, "Heel Lifts", as well as "Single Leg Bridges". To help with these you can also buy ankle, knee, or waist straps that attach weights to them. With weight lifting you want to over sleep because that's when your body wants to repair muscles that are torn during weight lifting. That's how they increase in size. It also involves eating lots of food with protein because your body uses that protien to heal the muscles. The more protein the bigger the muscles increase in size. You can also work out only 2-3 times a week with Weight Training because your not trying to burn more muscle than your body can heal with calories, like your trying to do with Cardio.
I wear a corset while exercising because it helps to amplify curves, and it actually makes exercises easier for me because of the back support. Try to avoid any exercise that works out the shoulders, or stretches them because TGirls don't want to encourage more broad shoulders. The arms are still lose muscle & fat tissue with leg exercises because the blood pumps throughout the whole body. Though taking prescription estrogen, and testoserone blockers does remove fat, and muscles around the shoulders, the bone structure stays the same. You'll have more slender, and petite shoulders. TGirls may want to avoid doing anything that makes them bigger though. Exercise has also been shown to increase testoserone, but as long as trans girls don't go extreme with it they'll be okay. If you just exercise enough to give yourself a little bit of a sweat, do stretches that don't excessively stretch your arms, and that's perfectly fine.