Very simple process

You take the following documents with you:
1) passport
2) I485 approval notice
3) H1B or other doc for your current status
4) EAD/AP if you have

The immigration officer will take your finger print and take away item 2, 3, & 4, then put I551 stamp on your passport. I551 stamp is as good as the plastic GC. It last up to 1 year.

The whole process is only about 5 minutes. But you should go there early as local office usually has long line. I got there before 7am and left at 11pm due to long line there.

BTW, before they let you go into the office, you will have a security screen that is similar you have at airport.

Good luck!