Like Lozange mentioned, the way you get arts is by opening the Bracer Notebook, looking up the requirements for certain arts and putting in quartz accordingly. For example, the AOE wind spell Aerial requires 4 wind points. In the prologue, you can get an Evade1 quartz that gives 1 wind point and an Impede2 quartz that gives 3 wind points. If both are put inside the same orbment, you get access to the Aerial art, but ONLY if both pieces of quartz are part of the same line. (the center slot in an orbment is part of all lines, so it's good to put the piece of quartz with the highest elemental values in there so all lines can make use of its points)
That's why an orbment with a single line is good (all elemental points are thrown onto the same heap, which gets you access to a lot of arts) and an orbment with lots of shorter lines is bad for spellcasting.
Since you cannot install multiple copies of the same quartz into an orbment, or even multiple levels of the same quartz type (such as Attack1, Attack2 etc), the amount of arts you can initially get is limited since the lowest tier quartz only gives you about 1 elemental point per piece. You won't be getting many advanced arts until you get access to higher level quartz in Chapter 2. Until then, you can get Aerial by putting Impede2 and Evade1/Shield1 quartz on one line or Hell Gate by using the Blind quartz with Action and Ep Cut quartz.
Estelle's orbment is element-neutral, but her lines are shorter than Joshua's. Of the two, Joshua has higher magic attack stats than Estelle, so if it's just the two of them, I usually give Josh access to offensive spells and Estelle access to healing arts, which ignore the ats-stat and heal for a set amount of damage.