Dieter, I think the answer is "very difficult!", and probably too difficult for anyone who has little experience of alloy fabrication. A big "come-long" with a strong wire was used, and a tab was welded to the doubler for hooking on the wire, in such a way that the wire could be un-hooked after the job was done. The doubler could be greased, or lubricated with slow-setting epoxy. The only penalty for failing to get it up into the required position is that the excess would have to be cut off and the idea abandoned, so it is worth trying on a small mast, but too difficult on a large mast.
In any case, it is debatable how effective the doubler is, since a tube that has been cut longitudinally loses many of its properties. I would say, just choose a tube that has the required properties without needing a doubler.