Each culture has a list of 6 background "packages" that players can choose from. Experienced gamers are encouraged to get creative and use these backgrounds as a baseline for creating their own. The formula for creating a background is quite simple, and I'll discuss it here.
Overview - What does a Background give you?
There are 3 big choices that you make during character creation that help to define your character. These are your culture, your background, and your calling, and each provides its own set of benefits. Your Background is what determines your basic attribute scores, it grants you an additional favored skill, and provides a list of Distinctive Features (Traits) that you can pick from (you pick 2 out of 8).
The math behind the generation of attributes is mercifully simple. You have a total of 14 points that you can split between the 3 attributes (Body, Heart, and Wits), with each attribute having a value between 2 and 7. For example, some combinations are (5, 7, 2); (4, 6, 4); (5, 5, 4); (5, 6, 3); etc. Some backgrounds will be more polarized (an example being 7, 2, 5), whereas others are more well-rounded (5, 4, 5). While there's a lot of room for customization, there are also patterns that each culture tends to follow. If you'd like a decent score in a weak area for a culture, you'll probably want to go with a well-rounded array.
Bardings: Generally speaking Bardings have a high Heart score, with the lowest value in a pre-selected background being 5 (most are 6 or 7). They also have a fairly solid Body score, with the lowest values being 4 and 5 being the most common. Wits is generally low, with the highest value being 4 and 2 or 3 being most common.
Beornings: Not surprisingly, Beornings favor Body with no value being lower than 5. Their Heart isn't too shabby either, with values ranging from 4-6. Like the Bardings, they tend to have low Wits, with 2 backgrounds each for the values of 2, 3, and 4.
Dwarves of Erebor: Their high values in Body are reminiscent of the Beornings, but their secondary attribute is Wits (from 4-6) rather than Heart, which is their weakness.
Elves of Mirkwood: Elves have an overwhelming majority of 6s and 7s in Wits, their highest score. Only 1 background goes as low as 5. Body is well-represented also, with mostly scores of 5 (though a range of 4-6). Like Dwarves they have low Heart.
Hobbits of the Shire: Not unexpectedly, Heart is their biggest strength with Wits in second place. Being so small of stature it's no wonder that they tend to have low Body scores.
Woodmen: Here is another culture that places a high value on Wits, as it tends to be their best score. Existing successfully so close to the Shadow of Mirkwood must also require a lot of Heart, because Woodmen have plenty of it. Relatively low Body is the price they pay.
You gain one additional favored skill that is in some way related to your background. As far as I can tell there's no strict pattern here, though there is a tendency for the skill to be one that the culture has 1 or 0 ranks in. Still, there are enough examples of skills that a culture has 3 ranks in that I'm hesitant to make any kind of general recommendation.
Your choice here will be pretty important, because you'll be invoking traits to earn Advancement Points or to gain automatic successes quite a bit. I don't have enough play experience to say which traits are flat out better and which are fairly weak, and in any case the list is too long to go over in detail here. It's mostly a matter of creativity anyways, and I personally prefer to pick traits that demonstrate qualities of the character that I have in mind. Because that's the whole point, after all. A player has this idea in their head for a character, and not only having those traits written down, but invoking them for mechanical benefits, is a great way to communicate these major qualities to the other players and the LM.
That said, certain traits do seem to be ubiquitous for a given culture, and I would therefore recommend that any custom backgrounds at least place these traits on the list of choices. I can't tell without a much deeper examination whether there are any traits that a given culture never possesses, but I would imagine that's pretty flexible, especially if there's a good reason why that trait should be tied to the background. Anyways, what follows is a list of traits that each culture pretty much always has. Any adventurer from this culture should have the opportunity to pick these traits.
Bardings: Adventurous - Your spirit is attracted by new experiences and challenges, especially when they seem perilous enough to put your mettle to the test.
Beornings: Grim - Your countenance in threatening, and betrays the harshness of your spirit.
Dwarves of Erebor: Wilful - Your confidence in your own judgement makes you deaf to all counsel but your own.
Elves of Mirkwood: Quick of Hearing - No sound escapes your attention.
Hobbits of the Shire: True-hearted - You are sincere, and your words and actions show your honest intentions.
Woodmen: Bold - You trust your capabilities to the point that you are not easily daunted, readily placing yourself in danger.