Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Sona, not much to her other than insane support capabilities. A lot of the builds that I have seen for her I noticed only focused on improving her ability power. The down side to those builds is that she remains extremely squishy. If you couldn't tell from the title and the build order, I recommend a slightly different way to play her. Build her improving her ability power as well as her defenses. Granted this build is focused on a balance between armor and magic resist. Most teams you encounter will have close to a 50/50 split in ranked and higher level play.
The runes I chose give you more survivability early game. From that point on in your game you can buy the items you need to compensate for the "lack"(according to some) of ability and damage output.
Pretty straight forward with the masteries. Basic utility/offense build for the cool down reduction and improved flash/clairvoyance.
The philosopher stones are only there for early to mid game phase. As a support you do not want to farm creep. Let your partner get as much creep score as possible, due to your lack of farming you will need the gold per 10 seconds increase to compensate. The only alternative is that if you find you self too squishy in the beginning then replace one stone for heart of gold.