TWIC stands for "Transportation Worker Identification Credential" and every Merchant Mariner must get one. 

To acquire your TWIC, go to this website to pre-enroll in the program: 

https://twicprogram.tsa.dhs.gov/TWICWebApp/

This will expedite your experience at your nearest TWIC processing center.  You must arrive with the necessary id, a means to pay, and a copy of your pre-enrollment form.  See the link above to see what id is required for your particular situation.  Payment must be made with credit card (Visa® or MasterCard® only), money order, or certified/cashier's check.  The cost is $105.25 if you want to tie the expiration date of your TWIC to your current Merchant Mariner license and $132.50 if you chose not to tie the expiration date to your current license.   When you pay the full amount, the TWIC is good for 5 years.  My license was to expire in two years at the time of the TWIC rollout so I opted to pay the extra $27.25 for example.

Using the link above you can pre-enroll and schedule your appointment.  I arrived and was out of there in 15 minutes.  You will sign in, sit down with a rep, show your id, and pay your fees.  They will then complete your application process with electronic fingerprints and a digital photo.  You then confirm all the data is correct and they submit the information.  You will be called or emailed when the TWIC is completed and you must return to the same center to pick it up in person.  Once again, use the link above to schedule an appointment to pick up your TWIC card.  You will be required to show your id and they electronically check your fingerprints.  You will also have to create a pin number to complete the card activation process.