Blizzard has posted an illustrated tutorial for their new Battle.net launcher, which was required to install Reaper of Souls and is quite an improvement over the old Diablo 3 launcher. (It integrates all your Blizzard games, saves you from having to log in or use your authenticator more than once a month, shows you which of your friends are online in which games, etc.)
You don’t have to use the launcher now, but you will when the Diablo 3 v2.0 patch hits, so you might as well get it now? If you’ve got it or not, when the v2 patch goes live there’s going to be a tool running to optimize and improve your Diablo 3 performance, probably by streamlining 2+ years of patches and hotfixes into one more coherent and cohesive whole.
On patch day, you will also have to go through a code-optimization process similar to what’s outlined Step 1 above. Expect it to take 15 to 60 minutes or so, and keep in mind your computer might run a little slower than usual during the process.
Check the article for the full details, and many tutorial pictures.