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.
Skill sequence = Early game is mostly ranking up your heal aura allowing you to lane longer and take care of your lane partner. Movement speed aura and DMG/AP aura, you can choose depending on whether you need to escape more or harass more.
For items, in the end if you ever get there, you should have the Innervating Locket, Boots(Depends on what you need for the other team), Deathfire Grasp, Soul shroud, Rylais Crystal Scepter and Shurela Firecrest item( dont know how to spell, thats what the philosopher stone is for plus it helps you make money since Sona is a bad farm character. If you'd like, you can also try getting the mejaj's soulstealer early game if your team is killing alot. It's a good item since Sona gets many assists and that builds up her stacks but if your team isn't doing so well, it'd be a waste of gold.
For summoner spells, I get ghost and flash, just because it helps you get away from ganks and get to team fights faster. Sona can get ganked pretty easily since she's squishy and has not many escape skills so ghost and flash help out alot.
That's pretty much it. peace :p