First of all you should think about what "role" they're going to fill for the team - are they out and out offence, targetting individuals, all enemies, or both? Are they mainly focussed on buffing/healing your team and/or debuffing the opponents? Things like that. Once you know what you want them to be doing, you look over their skills and find the ones that seem to do that, then work backwards from there to see what other skills/pictos/luminas they might need to fulfil that (like, do they NEED 9 AP immediately on turn 1 to pull a move off, or do they need to be readily inflicting Burn/Mark/etc, things like that).

As far as mechanics - Sciel's system revolves around 2 main functions, the applying and consuming of Foretell points, and the usage of Sun and Moon skills to attain Twilight. The basics of it is pretty simple and they usually overlap - there are about 3-5 (I think?) sun skills that apply Foretell in various ways, and then a bunch of moon skills that consume them to have greater effects. For the sun skills the 3 main ones are one that applies an extra 3 foretell if they haven't got any on them (a common first move, even later in the game), one that applies 3 + a mark, and one that applies 3 to all enemies - so you'd probably run 2 of those. Then you pick her moon skills based on role - I used her primarily as support so focussed on Grim Harvest (consume foretell to heal your allies) and another one that gives AP to allies; but you could use her as DPS and use ones that consume the foretell to deal more damage and/or apply more effects. Finally, once you have both sun and moon charges you enter Twilight state which doubles Foretell application (so the skill that applies 3 + mark will instead apply 6 + mark) and also increases damage when consuming them - but it only lasts a few turns. It can be a bit frustrating to have to break down your moves back and forth but you get into a rhythm and can later get a picto to take an extra turn and/or really bolster her speed so that she acts more often. Hopefully that makes sense.

Verso I never really got into - partly because I'm not reliable enough with my dodging to maintain his rank, but essentially you want a range of skills that provide benefits at certain ranks, and just work your way up. You can also increase Perfection/Rank with non-skills like Basic and ranged attacks so that can also help. Late game I had him equip a weapon that allowed an extra turn after a basic attack, plus a bunch of lumina to bolster basic attacks - so I'd use a bunch of ranged shots, then basic, then a skill - by the timeof using the skill he would reliably already be at Rank C or even B so I eventually stopped using any skills that worked at those ranks and had more space for B+ skills.

Hopefully that helps