It's a fairly easy makeover for anyone who's reasonably handy around their bike. The first step is to determine whether your downtube shifters are clamp-on or braze-on because you will need to install a downtube cabletop(s) at that location. There are cable stops designed for the braze-ons if that's what you have and also clamp-ons that would go in the same location as your shifters. The bar-end shifters are easy to install; first remove the levers and then tighten them into the bar ends using the allen-head set screw but remember it is reverse-threaded so you have to turn it left to tighten instead of right. Once that's done you'll need to remove your bar wrap to install cable housing from the shifters to the new cable stops and reinstall the wrap or put some new stuff on. No need to buy tandem cables because standard road bike cables will fit just fine. If you don't already have the bar-end shifters be sure you buy the more modern version that will work with indexed shifting. Here's a photo of a redo of some bar-ends on my Gitane Tour de France:



It's got a clamp-on cable stop on the downtube and you can see how the first several turns on the drops secure the cable housing to the bars before they exit to the cable stop.