Brush flux across the copper foil where you will be working first, then unroll a length of solder from its spool. Holding the soldering iron like a pencil in your dominant hand, and the spool of solder, with its length unrolled, in your non-dominant hand, gently touch the solder and the soldering iron together just over the surface of the copper foil. As the soldering iron begins to melt the solder, allow it to flow onto the copper tape, then move the soldering iron and solder together across the length of the copper foil so that the solder flows smoothly and evenly to cover it. Carefully rotate your glass lens in its clamp as needed (it’ll be hot!), and add more flux as you move to a new spot of foil or the old flux burns off.
You can easily add more solder to a spot on the foil, or use the soldering iron alone to remelt and smooth or redirect a lump of solder already on the foil. Soldering is actually a very forgiving craft, so continue until all the copper foil is covered, even those slight overlaps on the front and back of the lens, and you’re happy with the overall look.