However, another hard part is that you have to figure out how to make all their backstories mesh with the plot you've come up with. You'd think this would be easy, but I don't have any control over what kind of character they choose, what they look like, how they act, or where they come from. Well, that's a lie. I can tell them "no" to outrageous, stupid things, like traumatizing pasts no one should live through or warn them away from personalities that would get them killed.
This last week I came up with a backstory to incorporate my monk/Fortune's Friend into our campaign, and it is recorded thusly (it is not well written and is only a rough draft):
So. there was this monk named Dayton, and...during his travels in the world, he met an elf named Nahele. Actually, her name was a lot longer, but that's what he called her. She was a priestess of Ehlonna
they fell in love and wanted to spend their life together, blah blah blah.
However, dayton also felt a very strong tie to the monastery that raised him. He was one of the few monk/priests there and was one of the strongest, if not the strongest monk they had produced in decades. He felt it would be remiss in his duties if he did not go back to the monastery to teach the little ones.
This wouldn’t be a problem, except that Nahele was still completing her priestess training and couldn’t leave her forest yet. She was very obviously favored by her goddess and the high priestess of Ehlonna predicted many adventures and great things to be accomplished by Nahele's hands. However, the young elf maiden, very full of love...decided to follow her monk...at least for a little while. But it would be five years before they would meet again, and at that time, she would come and live at the monastery for maybe a few years. After that he would serve as her personal bodyguard while she accomplished her tasks.
During those five years, Dayton became an incredible warrior and an exceptional healer. He was an example to all those he taught. Nahele learned quickly in the ways of the forest and of healing.
After five years was up, Nahele made ready to go see the love of her life. However, the day before she was supposed to leave she became suddenly ill. Something that none of the priests/priestesses could cure
Instead of Nahele showing up at his doorstep, Dayton received a letter detailing how sick she was. However, he was not a very good scholar in elven and took the letter to mean she was dead, or at least close enough to it that it wouldn't matter. Barely a week into when she should have been at the monastery, an epidemic broke out at one of the local villages. It was a strange one, one that resisted much divine healing and drained an unusually high amount of energy from the healer for very little result. Dayton, overwrought with grief and determined to help in anyway possible, went to the village and tried to heal those that he could. Had this been a normal plague, he could have saved many. But it was a cursed thing, and it drained him of much energy. Even then, since he was so strong, he could have survived, but in his grief and desire to meet his love again, he overtaxed himself to the point of death.
(if you so desire to make this apply to your story, he could have noticed her at the last still alive and untainted and he set up wards and such to keep her untainted and so other monks could find her, and that was his last act before he died)
However, Nahele was not dead. In fact, she made a splendid recovery about three weeks later. It was, unfortunately, too late to save her love. She controlled her grief, however, and became a splendid tool for her goddess