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An opportunity has came up to do some design work for a friends small fabricating firm, likely no more than an evening or two a week mainly just converting sketches produced by them into a CAD program, with payment on a per drawing basis produced as opposed to an hourly rate. They'll supply a licence to the software and it'll mostly be done at home.
Having never done anything like this before, what's the options of how to keep this above board and manage this from a tax perspective, alongside my full time job? A quick google has left me more confused than I was before I looked.

Just keep a record of your income and necessary expenses, miles travelled, and register for self assessment. Fill in earnings up to 5 April and that's it. No need for a separate bank account or vat registration.