Then with AFI rating fresh in your licence you need to find a school which will supervise your instructing for at least 100 hours of instructional flying over a minimum of 10 months. You can then take a further test to get your full FI upgrade.
There are a few other detail requirements, best is to contact the BMAA for full information.
<http://www.bmaa.org/FAQ.asp?FAQID=2&FAQName=New+to+the+sport%3F&FAQEntryID=4>
<http://www.bmaashop.org/acatalog/Pilot_Training.html>
Good luck - it's a fun job.
MB