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.
The point of this build is to minimize Sona's main weakness - survivability, and enhance her greatest attribute - the ability to get a ton of assists. From the very beginning Sona has been an assist monster, getting an assist any time she is in a team fight due to her team aura spamming. A lot of assists are gained just by pressing e and helping your team catch up, while you never enact a point of damage. Most Sonas will gain 20+ assists a game.
To take advantage of all the assists Sona gets, this build gets 2 of the best stacking pieces of gear in the game - leviathan and Mejai's. The problem with these items is that death makes them ineffective. Tank build and runes helps take care of this. Pretty much everything that's not in her item build is designed to keep her alive. The three keys to staying alive are 1. having high movement speed, 2. having escapability, 3. having high resists. The runes and mastery points are designed to maximize survivability, gaining full movement speed, full resists, nearly 10% dodge, and 77 extra hp in the early levels, when Sona is most vulnerable.
Gameplay is rather common, and is always focused on survival. Use your easy-to-farm-with Q ability to farm minions early, use your passive to harass, and use your heal sparingly when necessary. The beauty of Sona's huge range is you rarely need to be in harass range, so you should not take much damage early game. Keeping in lane until ~level 7 is typically no problem for Sona, and your first blue pill back should easily get you your chalice, gem, boots of speed, and tome. This should allow enough farming to allow you to complete your 2 stacking items and your boots, at which point you've got all you need to survive mid game and early late game.
After you have chalice, boots of swiftness, and 2 stacking items, you can join team fights and use your heal/speed/Q/ult to gain assists, while taking advantage of summoner spells/ult to get out of almost any situation. Teams will get quite frustrated with how hard you are to kill with your abilities/runes/masteries, but that will not prevent them from trying, often netting your team easy kills while they try in vain to kill you.
As your stacks go up, your hp goes up, and your heal becomes more effective, making you tougher and tougher to kill. The addition of Rylai's helps make your Q spell an effective slowing spell, which in combination with your e makes it tough for enemies to get away. I normally get the Abyssal for the MR, and then complete archangel's if the game continues that long. It's very common to get dual 20 stacks, and die 2 times or less in a game.