You know you're ready when you can honestly say to yourself "Yup can't do much more than this". Alternatively you know you're ready if you're getting 20+ on Topscore and 18+ on Achiever. If you're wondering "am I ready?" and are not a timid person by nature, you're probably not. If you are a timid person, its not going to be easy to confidently recognize when you're ready.
If all of the above sounds wishy-washy, its because this is really something that is very different from person to person. There is no set formula besides the practice scores bit above (20 or Topscore or 18 on Achiever ~ 22 of more on DAT)
As for what you do right after the test...celebrate. Even if you got lower than expected scores, just let loose and have some fun. You can come back with full focus tomorrow and study for 3 months before the retake. Of course if you're scores are good, you can celebrate for a little longer!