Hi Mojo,
Switching from an Umbrella to
Ltd company should be really simple. Send an email to your Umbrella company informing them that you are looking to leave (ideally give them sufficient notice to process any outstanding expenses) and ask them to issue you with a P45.
Thats it; I would advise checking where you stand with regards to IR35 to make sure you can claim dividends without having to pay ENI on them (there will be tonnes of people who can advice you about that) and make sure that you look into any insurance requirements your end client may have, specifically Professional Indemnity, other than that you should be good to go!