Izanami is an excellent ADC, especially for those who like to put pressure on their opponents, as well as being very mobile.
It helps a lot to gain an advantage with Wave Cleansing, it has high penetration, very good for destroying tanks, as well as casting slows to control enemies.
Passive - Death Draws Night: Izanami gains physical penetration and intelligence as the fight progresses, which makes her stronger. When battles are longer, this skill is very useful.
Skill 1 - Sickle Storm: she gathers the power of automatic attacks and gives you an increase in attack speed for a few seconds (at which point the 1v1 duel is for her!). During the skill, the attacks don't pierce the enemies or return, but they do cause more damage to structures, very good for pressuring the lane.
Skill 2 - Spectral Projection: Izanami launches a projection of herself in a straight line, which passes through walls and enemies, causing physical damage and slowing all enemies it hits. This is very useful for securing that kill or fighting from a distance.
Skill 3 - Fade Away: Izanami descends into the underworld and moves away from her current position and, while stealthy, she gains invisibility and movement speed. I really like this skill because it makes her harder to capture (hard for those playing against her haha).
Ultimate - Dark Portal: a dark portal is summoned at a target location that deals physical damage and applies bleeding. The enemies hit are silenced and slowed down as well, which makes this ultimate excellent in combat, very good for securing kills.
Regarding the order in which to improve skills, I generally prefer 2 > 3 > 1.
As for the build, I always think it's best to focus on items that increase critical hits, penetration and attack speed (the perfect ADC combo). Here are a few examples: The Executioner is an essential item, penetration and attack speed, plus extra damage. Hunter's Bow, to increase movement speed; Wind Demon to increase attack speed and give critical hits; Demon Blade... and a few other items that vary according to the situation.
I generally like her as an ADC, but she can vary a little depending on the composition of the team, going to mid for example. She's strong and fast and, in the right hands, she's a powerful ally and a great threat to opponents.