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.
So this is my build and I usually rape face with this build on Akali. Some things you could change in the build, depending on who you're up against.
For the items you can always get merc's treads or sorc's boots. Be sure to sell your long sword when you go to buy your guinsoo's rageblade, or instead of a rageblade you can always try for a madred's (if nobody on your team is building one). I always make sure to get my giant's belt immediately after my sheen because it makes for WAY better survivability and then I proceed to my lichbane. If you find yourself doing really well with akali I reccommend picking up a majai's soulstealer as early as you can in the game, and if you're super confident, try picking up a sword of the occult AND soulstealer. (I would love to see screenshots if anyone successfully builds both majai's and occy.)
As for skills, I'm not sure what order my skills actually go in, but I basically max out mark of the assassin first and cresc slash second. Twilight shround always comes in handy in any fight so I am sure to get at least one point in that early on.
Summoner spells can really vary with Akali depending on how you are as a summoner. I personally like to go with ghost and ignite, or flash and ignite. Other good spells to go with would be exhaust (for 1v1 or chasing) teleport (for getting back in the action fast or backdooring) or heal (for obvious reasons). Heal is a great spell for akali if you're just learning her it helps with not dying lol.
For my runes I chose health and ability power. Other good choices are cooldown reduction (great for any champ) dodge (depending on your play style I suppose) or go for damage and AP for more of a hybrid build.
That's my build, I hope it helps!