I usually skip trying to plant a begonia that's been grown as a houseplant or a windowsill plant, and toss a leaf (or leaves) into a terrarium instead. The new growth will be perfectly adapted to the tank settings! Often begonias not grown in a terrarium get a lot of shock from the change in light, humidity, and drainage. I've also seen a tendency for people to plant the crown of the plant INTO a really damp substrate, in which it will rot. Water sitting on it's leaves and crown for long periods (due to misting) is also a solid way to get rot. They like humidity around their leaves and roots, but in a really humid tank you want them in a loose substrate that drains so air can get to their roots as well - the substrates they come potted in will turn to mud, and their roots may not handle the change (they need to grow new ones), and if the leaves and/or crown are having an issue... that plant isn't coming back.
Other than the misting issue, tossing in leaves and letting them root will avoid most of thoseproblems. I also will take begonias, remove most to all of the roots (just the roots, don't mess with the main plant) and toss the whole plant on TOP of the substrate (don't put it IN the substrate!). Plant will root and orient itself correctly when it's ready, and then do it's best to take over your tank.
