USCG National Maritime Center
Credentials Forms - USCG National Maritime Center
TSA: Transportation Worker Identification Credential (TWIC™)
The above links are a good place to start.
You will need to do a number of things:
Document your sea time
Pass the Physical and eye exams, using their forms,
Pass a Drug test,
Obtain a Twic Card ( which includes a background check and fingerprinting)
Pass an exam
Get fingerprinted by the CG.
There are a number of schools that assist with this process and are approved by the CG to proctor exams, they will also help guide you through the paperwork.
Depending upon the amount of sea-time you can document and the gross tonnage of the vessels you've served upon they have varying levels of licensure.
You can document time served on your own vessel. Time on other vessels must be signed off by the captain
or master of that vessel.
You can take the course anytime, but usually the course has a 1 year time limit; meaning you must submit your application to the CG within a year of completing the course. I might begin by starting the process to obtain a twic card, then document your time.
If you read through some of the above materials and have any specific questions, I'm sure you can get some answers here.
Good Luck, it takes a little time and a few sheckles...but hang in there