This is straight up about getting a Tear ASAP, that way you can use your Q more often and between the PoM and Manaflow band you get significant amount of AD when they are fully stacked; then it's just about rounding it off with enough Crit and Speed so that you can do what Sivir does.
Storm razor as a 2nd item has a lot of synergy with Sivir, it gives the slow while sivir's passive (which btw is a pretty outdated and useless passive) that allows you to get a little more move speed to chase ppl down.
Sivir is for the most part a very simple champ, her faults are basically in the early game where she cannot use her best ability (Q) often enough to create pressure and also her auto dmg early is mediocre or average which often means she is going to lose most trades if it's just a stand there and do everything sorta fight cause she has ONE ability to do dmg early and everyone else has 2-3; so taking Conq or PTA is the crutch that she has to use to duel against ppl early; taking LT imo is kind of meh because it doesn't give her any sort of trade potential like PTA or Conq, it's very nice outside of lane phase and when you have some items but it's one of those things that is just kinda meh.

Anyway that's all there is to it; you take this sorta build in lanes where it could be tough for you to farm up, getting a manamune early is actually a pretty significant spike in dmg potential and it'll give you the mana sustain you need to last. OFC nothing is perfect, and there isn't any instant win scenario with this or any build, it's just something that helps that early game and makes it easier to transition into some of the more traditional things you are use to seeing sivir do