I bought this car with a bad transmission. I changed the transmission by pulling the old one out and bolting the new one in. I bolted up the torque converter, put the transmission fluid in, ran it and it worked fine for about a week. Then the flex plate broke, sheared off, broke into many pieces. I pulled the transmission out and replaced it with a new flex plate and put the transmission back in. Ran it and it lasted for about a week and the new flex plate broke again. Are there special instructions for replacing the transmission and what needs to be checked or the specifications needed.

This can happen for a couple reasons. If the bolts were not torqued to spec, they will loosen. When the bolts are installed, you should put some Lock-tite on them. The blue lock-tite is good because you can break them loose if you need to service it. But it's also possible that the bolt holes have become oblonged from stress that may have occurred the original time that the flex plate broke before you bought the car. If that is the case, your bolts may feel tight when you install them, but can still come loose under stress. But if you want to try another flex plate, be sure the bolts are torqued to spec properly and put loc-tite on the bolts.
Good luck!