Buy yourself a rulebook and learn the rules. You must be able to apply them competently, in a manner that is positive.
Initially referee at your club. Becoming a good referee depends, in large part, upon establishing confidence in one's self and in others that your judgements will be sound. Training and practicing your skills in a non-competitive situation where more senior fencers and referees can discuss your refereeing will allow you to improve your performance and confidence
Details of these will be published in The Sword and in the Announcements section of this web site These will give a better idea of what is expected of a fencing referee. In addition read the book published by Michael Thornton on refereeing and watch videotapes of competitions and compare your interpretation of the phrasing with the referees.
Another thing to remember is that our sport is constantly changing. Attending seminars will keep you up to date on rule changes which could change the way you fence.
Attend as many local competitions as possible. Be warned, however, that your first opportunity will probably involve inexperienced fencers, and their actions are often extremely difficult to analyse. There is no help for this; persevere and learn from each exposure.
Be sure to arrive before the first round. You have to warm up as a referee, in the same way as a fencer. You can't expect to referee in final before you have done the preliminary rounds.
Keep score, time bouts, check scoresheets, watch other referees. We learn by doing and by example, and a competition is the place to learn.
Once you are confident enough to referee in your club and at local competitions then referee at least the early stages of tournaments that you complete in.
Once you have got this far and are satisfied with your abilities then you should take the first of your refereeing examinations.