FWIW,
I always try to solder the feeders to each piece of rail before placing it on the layout surface; preferably on the bottom of the rail. I do this at the workbench en masse. Turnouts too.
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I use liquid flux and apply it to the feeder wire ("L") and the bottom of the rail piece.
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I then apply some solder to both until the solder is shiny. You can then place them together and heat the "joint" enough to melt the solder on the two pieces together. A little more solder added to your iron before doing this can't hurt.
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I always use an old toothbrush and some cheap rubbing alcohol to clean each joint afterward, a little over kill for the type of flux I use, but I do it anyway.
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If the rail is already in place on the layout surface, or I'm doing a repair, I follow Allan's instructions.
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Have fun!
dan
DanBrewer