If you want the short version, start by focusing on opening up Slots first so you can increase your EP pool and equip a wider variety of Quartz. You can't do much with one or two slots. HP 1 is a very good early Quartz for Tear (so you can save your Mira for things other than healing items) while also providing a ranged attack option. You can also get the combinations for Hell Gate (minor AoE, instadeath chance) for Joshua fairly early on and Aerial (mid-sized Wind AoE and Impede 2 grants a 50/50 chance to cancel enemy chants) for Estelle or Schera not much later. Attack 1 with its 5% STR boost on Estelle once she's hit Level 8 will also help speed up the process of grunt killing via her shiny new Craft.
the having a water quartz is good for healing early in the game and also early in the game i noticed a lot of enemies are weak to fire so having one fire quartz might be useful to have on each character at the start, just remember that joshua naturally leans towards time quartz since a few of his slots are time only and estelle can be used in anything
mostly it's a matter of deciding if you want a large variety of low level spells or stack up and try to get high level spells by the same type of quartz in one line, for example putting all time quartz on joshua's main line
also finally remember that space quartz when combined with elemental quartz usually add extra spell be sure to check your bracer notebook to see what elements mix with what, like i put a space quartz, i think EP Cut 1, in estelle's first slot because it gave her time magic and combined with other quartz on both of her two lines
remember what the person above me said it's good to unlock lots of slots early to have more EP for casting spells and more room for either a variety of spells or for more power spells
if you can for joshua since he's already time based try unlocking the time spells shadow spear and hell gate as they are very powerful and useful and for estelle i just liked having her as an all-arounder to to heal and attack the enemies weaknesses with first level spells of a lot of quartz
Added note: your next party member, i won't spoil who it is, is wind based so giving that person a lot of wind quartz is useful... try unlocking Arial, it's an AoE spell and very powerful --- Currently playing: Megaman SF 3, Lunar: Silvery Star Harmony, ARF, & MMZ: Collection Green Dragon FC: 0387 -6219 -7134, Saurian FC: 1247 -0142 -9895