I'll refrain from my usual hyperbole and give you the straight assumptions that I have about the character funnels, having read the rulebook and a few of the published adventures. Let me know if I'm way off base.
1) You completely roll randomly for your ability scores, race, and occupation. (Or get a scratch-off lottery character.) There are no mulligans, adjustments, rules for discarding "bad characters," etc. You play what you roll, regardless if it's nonsense, unviable, or a living god.
2) You create a stable of these characters - usually around four of them (but that can vary depending on the number of players).
3) More than likely one hit from anything will kill your 0-level character. One failed saving throw will also very likely kill your character.
4) You've got no real class abilities (because you don't have a class yet). You try to survive on your wits and the luck of the dice.
5) If you think there's a monster or other danger in the room, throw in your worst character to be the meat shield. After that character dies, your more capable character can try to attack from a range or run away.
6) You get to the end of the adventure and get to "level up" your surviving character. Because he was rolled randomly, he might not be what you wanted to play and may not be useful to the rest of the party.
7) If you're unlucky enough to lose all characters in your stable during the character funnel, I guess you just sit out the rest of the time? Hopefully you get to level up another player's survivor.