To me, Ynnari building is all about story, which is why I personally like the concept.
But basically, there are only 3 Ynnari characters, so they literally must fight with soldiers from other Aeldari factions. There are two ways for them to do that, and which one you choose is dependent upon where YOUR headcannon Ynnari are in terms of their alliances with the other factions.
Yvraine has to convince CWE, DE and Harlies that her plan has merrit and that she can be trusted. She does this in part by walking the walk- fighting in solidarity with her brothers and sisters to earn their trust. To play games at this stage, add Ynnari Characters to existing units. Easy; DE continue to be DE, CWE continue to be CWE, and while Yvraine and her crew don't benefit from the Obsessions/ Masque Forms are Craft world traits, they don't prevent the rest of the army from benefitting from that stuff. Furthermore, even playing with the Ynnari in this fassion provides a story reason why detachments from both DE and CWE might appear in the same army.
Thing is, as Yvraine and her crew continue to be exemplars of the older, pre-fall mentality, they gradually begin to emerge as leaders and recruit people to their unique cause. And this is where things go a little crazy. Ynnari characters can take control of the detachments they join, creating up to 3 "reborn" detachments. When that happens, the CWE, DE and Harlies in the detachment basically stop being DE, CWE and Harlies. They lose their obsessions, masque forms and craftworld traits; they switch to Ynarri psychic disciplines, warlord traits and relics.
Unfortunately, what they gain by doing so is not remotely a fair trade. The Strength from Death ability is fluffy, but not a good enough power to justify everything that a player has to give up. It augments close quarter fighting somewhat, but it does NOTHING for units whose focus is ranged combat.
To get back to the story piece, here's how I'll be doing mine when I get there:
Yvraine courts the CWE
The Visarch coursts the DE
The shard of Ynead courts the Harlies.
Battles start at 500 points. If the Aeldari win, and the Ynnari units survive, they earn the right to lead in subsequent battles. They have to keep trying and fighting at this level until they earn that trust.
A Ynnari who has become respected enough to lead can also allow his or her patron faction to ally with any other Aeldari faction which includes a Ynnari, whether or not that Ynnari has also earned respect. In this way, people struggling to win at the 500 point level can be bolstered in its efforts by other Ynnari.
Once all Ynnari get their respect, that's when armies grow- first to 1k, then 1.5k, then 2k. The final army is 1-2 each of Harlies, DE, CWE and all 3 Ynnari, and you play it at Apocalypse scale.
Now in terms of list building, there are some cheats. The best of these is DE Mercs, meaning Incubi, Scourges; Mandrakes, Solitaires, named characters including the Avatar of Khaine and <Coven> models can't be in Ynnari detachment. These units NEVER get Obsessions, can't take DE warlord traits or relics. This means they have NOTHING to lose by turning Ynnari. Ever wonder why both Incubi and Scourges were included in Blood of the Phoenix? Now you know.
So that's where I start. I love Incubi; I always have, and they are WAY better now than they were. Ynnari have a special strat which allows you to grant Strength from Death to up to 3 units of Incubi, so it's good to take 3 units so that you get the most out of your CP; in fact, now that all members of the unit have Lethal Precision and not just the Klaivex, maxed out units rather than MSU are also useful.
Add in two squads of 5 Scourges for mobile shooty, and that's a solid Battalion.
As for Yvraine's unit, she needs to bring some shooty from the CWE list; she also really likes it it when Aeldari psychics die near her, so I'd recommend a psyker as the second HQ for this battalion if you need a second HQ. Maybe some Dark Reapers? Aesthetically, Swooping Hawks be a nice mirror to the scourges.
The Yncarne will have a fair number of bikers, and there's a strat that lets Ynnari bikers Advance + Charge, so that's one strategy for engagement. I don't have the Harlie dex yet, so I'm not sure how supercharged their bikes are, but the Quins are close combat specialists, so this should be somewhat viable? I don't know how strong they are without their special masque forms and stuff.
Another recommendation to make a Ynnari army feel more Ynnari is to create your own Ynarri Reborn character. Pick any generic HQ choice from any non-Ynnari Eldar faction; better yet, let your story pick one for you- the first character to "rescue" or be "rescued by" a Ynnari model in a game is the perfect candidate for this. Give it the Exalted of Ynnead strat, and it gets to emulate a Ynnari WL trait. Then take Artefacts of Death to give it a Ynnari Relic. Then over time, convert the model from its original to its reborn self.
Given the baroque nature of Ynnari models, there's lots of conversion fodder in the AoS range, and more on the way with Lumineth. Toying with the idea of throwing in a Griff Hound from the Fantasy range on another Reborn Character's base to continue the aesthetic of the Gyrinx. Unfortunately the Gyrinx sitself seems to only be available through Yvraine; Griff Hounds are bulkier and birdlike as opposed to catlike, but it's the closest you can get.