Easiest way is get an accountant to do it for you, takes all the guesswork out of it and they can give you advice if it's best to have a limited company or as a sole trader. Typically they'll take an off-the shelf company from a company who specialise in this then rename it and do all the paperwork to transfer it to your name. You'll probably want someone set up as the company secretary who is responsible for doing all the returns to Companies House. Any good accountants will have a package for such start ups and can also do your year end accounts, company returns etc. at a reasonable cost.How you do it?
Good tip, always make sure you sort HMRC for tax & VAT before anyone else. Don't be greedy, be honest and you'll do well. Don't neglect your pension, easy rule of thumb is every £1 you earn, expect to see no more than 50p for you and you'll not go far wrong.
For the IR35 piece if contracting to a single customer, you could also consider looking at umbrella companies but do some research as there are good and bad around