Done dozens over the years. Uni bellows, bell housing first, then some glue on the gimbal housing flange, push the bell housing hard into place, beyond where the hinge pins would be holding it, and working from the inside the bellows, push them up onto the flange. Once the uni bellows is on, put the exhaust bellows on the gimbal housing, then using the tool, pull them onto the bell housing flange.
Yep - like Christ mentions - that is the best sequence. But as Dave says - the clamp position is pretty critical. You will need the manual to see where to position the clamps.
If you are doing the water hose also you can put the end in a pot of boiling water (not touching the bottom) and it will slide on like butter. Make sure you have a screw driver with a flex shaft. It makes it easy to get to the clamps.
My tip- do it all while you have it apart. 3 bellows, water hose, intermediate shift cable, gimbal bearing, gear oil and waterpump. Nothing worse than re-sealing the transom, and having your shift cable go on your next outing. Get a copy of Mercs #6 manual, follow the procedures, and either buy, borrow, steal, or fabricate the specialty tools that are suggested.
On the water hose, I have found it easier to remove the upper tube completely - remove the two bolts from inside and then remove the rubber bushing. The tube will come out and the hose can be put on with the bend in the correct orientation. In my opinion it is much easier that way. You don't have to wrestle with the hose and clamp in a tight spot.
