Almost everything that goes wrong with self-employment tax goes wrong in the first year, and usually before any tax is even due. People register late, or register when they did not need to, or assume that because no money is owed yet, nothing needs doing.
This module deals with the three questions that come before any maths: does HMRC actually need to hear from you, what does the £1,000 trading allowance really do, and how do you get on the system without tripping a penalty on your way in.