Once you get the threads going, put one hand on the back of a tank and gently move it one way or the other to allow the manifold to rotate freely. Observe the distance between the lock nut and valves, both sides of the manifold should be screwing into their respective valve.
You will only be able to screw in the manifold about a 1/2 turn or so before you have to adjust your parallel. It is important not to stress the manifold by pull/pushing too hard on the tanks. The length of the tanks gives you enough leverage to seriously damage the manifold if you are not careful. It's easy to be careful, just take it slow.
In the case of 104's, keep screwing in the manifold until the distance between the tanks is just a little wider than 1 bolt width. You want both sides of the manifold to be within a thread or two, no more or you risk a leak. If the tanks are not in parallel when the manifold is all the way in, don't panic, the manifold is designed to have a certain amount of give. Do not tighten your lock nuts yet.
The Bands
The first thing you need to do with your bands is to stretch them out in both directions. This will allow the bands to conform to the tanks and not pull them out of position.
Slip the top band onto the tanks. Start your nylock nut onto the all-thread by double-nutting one end. Run the nylock nut down leaving about 2 or 3 threads exposed. Do not hold onto the all-thread with vice-grips or such because if you bung up the threads you will regret it later on.
Run the all-thread up from the bottom to the top using a washer on the bottom and a washer with a lock washer on the top. Some people worry about the threads contacting the sides of your tanks and put some heat-shrink tubing around the threads. Me, I don't worry about it.
Put your second set of bands on the bottom and don't bolt them. Snug the bolt down on the top bands and observe the spacing between the shoulders of the tanks and the bottoms. Screw your manifold in and out until you have more or less the same distance at both ends. This is an important step. The tanks will pivot around the top bands. As your manifold contracts the distance at the base will increase. Finalize the position of your isolator valve and snug down your lock nuts.
You will notice that one of the lock nuts has some marks on it. That signifies that it is a reverse thread. This way you lock down both nuts by turning them in the same direction.
