Transmission output shaft has a flange as does your prop shaft (the rubber coupler may be between them to absorb shock).
Put engine in neutral so you can easily rotate the shaft. Bring these two flanges as close as you can get to each other. Loosely put in the three bolts holding them together.
Get a feeler guage. Rotate the shaft and use the feeler guage to measure the gap between the flanges to insure it is constant all the way around. Adjust the engine mounts to adjust the gap. For example, if the gap is too large at the top, the front mounts need to come up.