1. If necessary, decorate the flat to make it more attractive to potential tenants.
2. Get a qualified electrician to carry out a safety check of the wiring, sockets and any appliances you'll be providing.
3. If there's a gas supply to the flat, get a Corgi-registered fitter to check the safety of appliances.
4. Read the booklet which can be downloaded, either as a pdf or as a Word document, from here:
http://www.communities.gov.uk/index.asp?id=115 1898
5. Do some research to decide on an appropriate rent.
6. Decide whether you wish to charge a deposit (and, if so, decide how much).
7. Advertise the flat in your local free-ads paper (or wherever else people, in your part of the country, look for accommodation).
Chris