You basically open the doors and go. The temptation to operate this way is great and many succumb. The shortcomings of this form of business are great also. There is absolutely no protection between you and your business and there are no tax benefits. There are no particular record keeping requirements which is quite easy but often leads to sloppy business practice.
Sloppy business creates poor decision making and sets up failure.
At the very least you become a sitting duck for IRS audit and lawsuits. Planning, at the beginning, can save you money on an ongoing basis and protect the assets that you have worked so hard to accumulate.