All the info you need about becoming a FISO is documented on the CAA's site :
www.caa.co.uk/default.aspx?categoryid=43&pagetype=90&pageid=4748
Essentially it's the written exams, then 40 hours minimum training before a validation wither with a CAA ATS Standards Inspector or a Unit Examiner.
Following that there is a competency check every two years, plus whatever checks the unit you work at choose to give you as well.
In addition you'd need to provide FIS at least once every 90 days to maintain your validation.
Whilst the written exams are self study, I'd suggest getting in touch with the unit you hope to work at first - they may be able to offer some training, and more importantly confirm that they'll let you validate and work there once you have the writtens.
Good luck
