First of all, to anyone interested in reinstatement (in response to a couple of threads about the subject, and for anyone that wants to see their family again), there is no formal letter to write. Rather, you will have to submit a request to the hounders that disfellowshipped you in the first place. Chances are good that they will reject that appeal.
Some time later, you will have to submit another request for a reinstatement hearing. At that time, you will have to display "works befitting repentance". You will need a recent history of attending most of the boasting sessions, you will need to discuss your personal study habits, and you will have had to stop doing whatever you got disfellowshipped for (or at least fake it, but they will have to believe you or it will not work). At which point, they will schedule a hearing (and possibly drag their feet as long as they feel like it). At that hearing, you will have to present evidence of repentance from whatever you got disfellowshipped for, along with good study habits and boasting session attendance.
If, after the hearing, they are satisfied, they will reinstate you effective when the next circus meeting is. They will announce you as reinstated, and you will once again be able to speak with friends and family (though there is a good chance that they will want to see further proof that you are in for good this time before they draw close to you again).
Now, the pitfalls. After a reinstatement, there is a good chance of being on probation for a time. During this time, you will be subjected to hounding calls and interrogation to ensure you do not get a chance to slip up. They will be more likely to keep a close watch on your boasting session attendance and field circus regularity (you will not be allowed to comment or pio-sneer, but they will want to see you at all the boasting sessions and out in field circus at least 10 hours a month). And, any "leaders" toward the sin for which you got disfellowshipped are going to be watched closely--if you went out for fornication, you will be monitored for any songs leaning toward sex, suggestive videos, or visits to web sites that are slightly racy (even some department store ads contain items that could be classified as "racy"). Of course, if you got disfellowshipped for apostasy, you will be watched for any Internet access, possession of other religious materials, and any straying from what the Filthful and Disgraceful Slavebugger is teaching.