I’m going to be honest.
I’ve read this thread and I think you need to use a credentialing service. You pay them a fee and they file for you.
Then call sea school and tell them you need to take STCW basic safety training.
All the instructions on obtaining a credential is on the national Martime website. You just have to read.
Using a credentialing service they will tell you what to do any don’t have to read.
At any rate you will not go to work immediately unless you work inland.
A credential is going to take 4-6 weeks and the stcw class is going to take maybe 10 days, which you need before you apply for a credential.