If I'm on a boat where I have room to walk around and can walk to a platform and giant stride in then I place the cylinders on the bench, one on each side. I clip them in place, bungee around the valves, hoses where they belong. Fins come on and I walk to the platform and jump in. I've done this with 108s, 2 AL40 deco cylinders, and my SS Magnus with no issues.
I'm very new to sidemount, but this is my procedure as well. Kit up except for tanks early on the boat ride. Tanks and fins when the captain tells everyone to get ready. Giant stride in with both lp108's clipped and bungeed. This is what I learned in SM class, and successfully executed on subsequent dives. Fins are a little difficult to don with tanks on but it is doable. I don't think I'd like to do it the other way around (fins then tanks).
Shore diving sidemount? Well.. I'll just say I think I have some issues to work out and nothing constructive to offer .
I might have to try that "drop the tanks first" method posted in this thread. If I can find a willing boat op. It sounds nice.