I'm not too sure on how you can become Animagus, I was born with the ability. I know though that if you do posses the ability, then it takes a lot of concentration and focus on your magic to shift. 

I used to turn into an owl that had a torn shirt on, or my robes swill owed me whole. You need to focus on your clothes as well so that they turn with you. It can be frustrating at first, but with time you will get it. 

I had to focus a lot to be able to shift at will. At first I did it unconsciously at random times, which was very embarrassing. I knew I had to do something. 

So I meditated. I focused on my core, my animal, my magic, the pattern of my breath, the rhythem of my heart. And with patience I eventually was able to master Animagus form.