It would be hard work with a bad knee I hope you are fully recovered. Start off by subbying this is what most do, it will also give you a feel of whether you are still able to do the work.
Working for yourself is not for everyone, if you have no work you get no pay, if you are off sick you get no pay, if you want to go on holiday yo wont get paid while your off unless you have been sensible enough to include this in your labour charges.