Something you should know on the outset: this form is for non-Hague countries. The form of this kind for Hague countries is the I-800A, and I have no experience with that one. So there's that.
Now, I realize I should probably make my next post on acquiring all the supporting documents you need to file various and sundry things (birth certs, marriage certs, etc), but just imagine you've already gotten those.
Your I-600A is an exciting document. It's your request to the government that they approve you to adopt from the country you specify. I think thing #2 you should know (thing #1 having been what the doc even is) is that this thing takes for-ev-er to be processed. As in, if they get bogged down, it could be 3 MONTHS before you receive your determination. That determination is called the I-171H, the "notice of favorable determination for advance processing of an orphan petition."
But I'm getting ahead of myself. Even though your agency should send you instructions on how to fill out the I-600A, always check with USCIS for their latest incarnation of this thing (and fees, etc). You know the government.
With these couple of sheets of paper on which the future of your family depends (nbd), you will submit the supporting documents: birth certificates/naturalization papers/passport copy, marriage license/divorce decree/death certificate. These documents can be copies, but you should know that newly-issued Texas birth certificates do not copy. As in they literally have that written all over them when you try to black and white copy. And of course you'll send a check. The instructions at USCIS will have the current fee. Don't forget to add the fingerprinting fee required for each adult in the house. I should tell you, this puppy is one expensive document. Our I-600A, with fingerprints, will be $890.
The directions are, of course, not really THAT clear for being so danged wordy. For example, on the current incarnation, they don't really tell you what to write on block III if you aren't requesting an accommodation for a disability. Apparently you just leave it blank. I put "no" all the way down... We'll see if it works out okay... fingers crossed.
I'm told that at some point, we will get an appointment to get our fingerprints taken. I don't know why our fingerprints need to be taken and taken (I mean, Stephen and I were both fingerprinted when we were hired at GCM, and I was fingerprinted when I was with our ISD, so come on, guys!). It's not like they change. But once that happens, I'll let you know how it goes.
For us, as soon as we get our I-171H, we go on the wait list at our agency. Which is kind of a big deal. Then, the REAL wait begins.