You will probably find that as you plan your design step by step, you will develop your own individual designing style quite naturally. Your personality is unique, you observe things differently to how others do and the muscles in your back are different to anyone else’s. So in a sense you might not be able to avoid developing your own style.
Developing you Own Designing Style
If you feel that you don’t have your own designing style then try experimenting with a variety of styles, techniques and media until you find a style or a combination of styles that fits you.
You can find examples of other interior designers’ work on the internet or at the library. As you worry less and less about your technique and you reach the point where you can just design you will develop you own style quite easily.
That said, it is important to be versatile as an interior designer so you should aim to have a variety of different styles in your repertoire. Don’t limit yourself to only one style as this may make it difficult to get commercial designing work and you will probably get bored.
It is important not to compare your manner of designing to that of other interior designers, as there will always be someone 'better' than you and someone 'worse' than you. Be inspired by others, but don’t compare.
Developing versatility in designing
Besides for developing an own designing style, it is always good to be versatile in your designing to help keep you stimulated and to ensure that your work is marketable should you want to design commercially.
There are various ways to become more versatile.
First is inspiration: You should expose yourself to as many different designing styles as you can by looking at art books and viewing exhibitions amongst others. You could also try taking classes from different teachers as each teacher has their own style of designing and teaching which they will pass on to you.
You could also be your own designing coach and set yourself different tasks with different goals, aimed at different audiences. For example you could set yourself a project in which you decorate a nursery, or design a manoir, or design a the interior of a boutique hotel. This will provide you with practice as well as material for a portfolio if you do want to get into designing commercially.
You must ensure that you continually challenge yourself by learning to designing different subject matter, using different media and different techniques. Perhaps look at websites and agencies that market the work of illustrators to see what clients are looking for so that you can develop your skills in the right direction.
All these combined efforts will finally guide you to learn step by step your own designing style.