Hello everyone, I am Jugales!
I am a Silver ADC main with in-depth knowledge on Kalista and how to play her successfully. I decided to share this guide to share tips, tricks, knowledge, and more on Kalista that I haven't seen much in other guides. With that said, I hope this guide can help. If you see any problems or have any tips, drop a comment below!

Early game with Kalista is pretty standard. You would like to focus mainly on cs while harassing the opposing adc as much as possible. If the opposing support is fragile, you may harass them as well. If you are against lane bullies, make sure to play back and be careful. Kalista's main threat early game is her ability to catch champions out when they are out of position. If any champion gets close, lay as many rend stacks onto them as possible. As soon as they are out of range, burst them down. You may use your sentinel to watch for ganks, sending one off into the river could save your life.

Mid game is when Kalista becomes the most strong. At this point, Kalista should have lifesteal and attack damage from Bloodthirster, and insane attack speed from Hurricane. If you have synergy with your support, this is also a good time to utilize your ult. Kalista's ult has a short cooldown compared to many others, and it can secure a kill against many champions. Tanky supports like Malphite, Alistar, and Leona can capitalize on this most due to their added crowd control. Mid game is also your chance to take down turrets and roam.

Late game as Kalista is very fun. At this point, you should have all of your damaging items (BT, LW, IE, BOTRK, Hurricane) and you can sell your boots for Zephyr. With an incredible amount of lifesteal, attack damage, and attack speed, Kalista has the ability to go against many squishy opponents at once and win. As Kalista, you can also use this time to focus baron. At full build, Kalista is able to solo baron. Watch your rend stacks on baron carefully, since you can use it as a smite. You may do the same with dragon as well.

Your early game build is the same as any of ADC basically. It's nothing special (besides your spear).
Early game, you want to rush Bloodthirster. When you have it, you are able to engage on the opponent more with higher attack damage in lifesteal. Your next item is boots, which will give you the added attack and movement speeds that you need. Finally, you will want Hurricane. Hurricane will give you the ability to melt multiple targets and wave clear very fast. With Hurricane, melee minions can be burst by rends after 3 stacks and caster minions can be burst after 2.
Late game, your item goals are toward crit, damage, armor penetration, and boosting your stats as a whole. Infinity Edge is essential because of the passive, damage, and crit chance. It is a great item and for a great price. If the enemy team is more tanky, you may want to build Last Whisper before Infinity Edge however. Once you have these two items, you can build Blade of the Ruined King for lifesteal and its active. Finally, you can sell your boots and buy Zephyr for many more stats.