How Can I Design An Extension?

Your question is not very clear. Hence it would not be possible to answer this question very satisfactorily. The word extension can mean several things depending on the context in which the word is used

The internet is an ocean of information and resources and you can get almost any information that you need from the internet. You just should be clear in your concepts and put in the right key words to get the proper results. If by extension you mean some technical procedure then it is always best to seek the advice and guidance of a technical expert to avoid trouble in future. But be sure about what you want thoroughly before to venture into anything.