Developing a personal yoga practice is not something a beginner should worry about. In fact, in the beginning it is advised to take some yoga style you like and follow it to the letter.
Because in the beginning you don’t know what works best for your body and how it works. Creating your personal yoga routines means you are able to feel how particular asanas or yoga poses along with breathing and meditation work on your body and mind. Once you know it you can adjust and adapt.
When you start your yoga, you hardly understand anything in terms of asana influence (both physical and subtle), so it is better to simply follow a trodden path made for you by the experienced yoga teachers and gurus. There is nothing wrong with it! You can do so very successfully and have no need in inventing something of your own. But sometimes the need comes when you have got some experience and a desire to make this change.
It cannot be done whimsically; it is done only on the basis of the real need you feel to extend and improve your personal practice. Start slow, and make little adjustments to how you perform your asanas and pranayamas. Well, actually, not much can be said on this matter, since it is all about your personal feelings and experience, so they will guide you through the whole process. But keep in mind, that successful adaptation of any practice is possible only when you have established a proper connection between your consciousness and your body. Your body will tell you then what to do.