Hi,
I have been playing in Glorantha for 28 Years, I have been illustrating Glorantha for 20 Years, I have been writing about Glorantha for 18 years, I have worked for Chaosium, Issaries, Moon Design, Mongoose, The Unspoken Word, Tradetalk and Tales of the Reaching Moon. I have written four 50 player Gloranthan Freeforms. And after all that I still don't beleive that I could claim to know 1/100th of the stuff there is to know about Glorantha. However I never fail to have fun with the setting, I love it deeply and its exploration is my passion. So just try to understand the bits that you need to, to a level that you can have fun with.
I would recommend that anyone playing in Glorantha read either Heroquest Voices or the 2nd Age Equivelant material in the Mongoose Books. That you only concern yourself with a few cults at a time. Keep non-humans as the weird and mystical, keep foreign powers the same where you can. Remember that if you take a small area that you can develope your own campaign around and then develop it your own way, rather than trying to tackle a huge amount of detail. If you want to create your own material, but use the Gloranthan material their are lots of little places where this can be done.
Some suggested places to start...
A small Orlanthi clan - It doesn't matter really if this is in Ralios or Kethaela, just create a small village or similar at its heart, some local woods inhabitted perhaps by local spirits, perhaps some elves or even a dragonewt. Then you need a few local hills, perhaps with the odd caves that are taboo, perhaps because they hold monsters or are of a religious nature. This is Glorantha so there should be at least one weird location from a forgotten era, a collossal face in the wilderness, a strange henge or monument or an bizarre ruin.
The inhabitants of this area need to be pencilled out, work out who the chieftain is, his wife, his champion, the local healer, a shaman or priest and perhaps two more local characters (a bully, love intrest or mysterious stranger).
Cults, keep it simple, Have most men worship Orlanth, most women worship Ernalda, then have the Champion worship Humakt, there will be most likely one follower of Lhankor Mhy, Chalana Arroy, Odayla or even Vinga. If you are doing a Draconised clan then you need to use Orlanth the Dragon and Ernalda the Scale.
You now need at least one source of conflict, a nearby clan, a troll tribe, nearby foreigners (Coastal Pirates, Praxian Nomads, or a small group of God Learners, is a good source). You will need to know a few protagonist from that enemy, their leader etc. You only need to know a little about this enemy to start with.
(To be continued)