Yes, there is a correct procedure for installing the 90-T trigger assembly back into the gun.
The safety button must be in the "OFF" position. In this position, the pin on the safety pawl(the little toggle on the back of the assy), will slip between the legs of the safety button.
If the safety is "ON", the small pin on the safety pawl can slip ahead of the front safety leg, preventing the safety to move to the "OFF" position. Also, because of the position of the safety pawl, the trigger assy will not come out of the receiver.
The safety buttom will need to be removed by accessing it through the back of the receiver. You will need to know how the safety assembles on the inside to remove the button.
If you're unable to disassemble, I'll correct it for you at no charge...you just pay for the shipping.
Doug