Here I'm going to talk specifically about religious cults, though some of the points will hold true for shamanic and sorcerous cults as well (I'll do them in other posts).
I have never given a cult more than three myths, and I try to keep it to one or two. Myths will give you a sense of the cult and its central concerns. Imagine you were designing Christianity as a cult in MRQ2. You'd probably want to start with the gospel narratives and work outward from there, rather than deciding that Catholic priests will have the following powers: Transubstantiation, Exorcism, Blessing and trying to work back to what kinds of stories they tell that justify those things. However, when it comes to fantasy religion, you often see things like "These guys are the priests of the war god, so they need lots of fighting powers, which spells are the best in combat?"
In general, I find it useful to plan one myth to be the life of an exemplary individual, and the second to be the actual teachings or interpretation of the religion. Exemplary individuals are ubiquitous in monotheism and extremely common even in polytheism. Heroes, demigods, saints and avatars all work well. The use here is to justify the specific set of behaviours associated with that individual's life, and to provide you with a prepared reference you can drop when appropriate (people might swear by the hero, their icons might be religious symbols, etc.).
I often use the second myth to develop how the ideals of the hero's life are interpreted or developed by the religion. I like to introduce some conflict or discrepancy with the first myth here. This allows for intra-religious debate, schisms, etc. and basically keeps the religion from being overly simple and dull. It also provides PCs with conflicting impulses that others can play on an exploit, or that the PCs can play on and exploit. I almost always make the second myth considerably (20-25%) weaker, for same reason that television is more influential on popular ethics than Aristotle is.
Having the second myth be the result of abstract debate makes heroquesting it easier but more interesting - you basically can do things like throw yourself a church council and claim you're re-enacting the struggles that defined the faith, which is way cooler than yet another myth where you have to punch out Satan or some dragon or whatever.
Myths are supposed to have their percentile score divided by twenty in associated behaviours, but I semi-ignore this. In general, the only kinds of associated behaviours that are worth marking out are ones that separate the person from ordinary society, and specifically, those that the players wouldn't automatically think are good anyhow. If members of your church are supposed to always tell the truth, well, congratulations but it's a total waste of time to write out. On the other hand, if members receive special dispensation to lie remorselessly to non-members, that's worth noting.
The one time you might want to mark out an obvious ideal is if it's ignored by most members of the religion in practice, but serves as a rhetorical point you can use to batter people who try to disregard it. Here it becomes a way of mimicking the rhetorical effect that religious condemnation has on premodern peoples. For example, "turn the other cheek" or "love thy neighbour" in a medieval game would be totally appropriate to note, so that priests can invoke these as the PCs plot their murderous revenge against the shopkeeper who refused to discount their arrows enough.
Heroquesting is something that notoriously gives people shitfits to figure out, both PCs and DMs. My recommendation is to delink it from myths. I know this is total heresy for Gloranthaphiles, but I stand by it. I recommend having very specific, real acts that people must do, rather than complicated mythic journeys that overlap worlds. In Moragne, you "heroquest" by going to very specific locations and stepping through to the Heaven-equivalent (Beyond the Veil). Once there, you meet God, and if you survive, you come back rune-touched. Nice and simple (plus it allows me to roleplay God). In the Dawnlands, you get runic associations through tons of different ways, including your daimon marking you, being soul-forged, dying and then reincarnating, etc. While many of these have religious components, they're not really classic heroquests in the Gloranthan sense, because while I am capable of running those, they strike me as more effort than they're worth, especially when multiple PCs of the same religion want the same runic associations.