Joe has given good info here.
Officially, you put your entry in to a drawing. There can be a few hundred or up to a thousand riders who enter the draw, and roughly 100 are chosen.
They try to have a mix of veteran IBR finishers, newbie riders, and a handful of those who have tried and failed to complete a previous IBR (unfinished business). They also like to encourage female riders and two-up teams.
Unofficially, if they don't know who you are at all, then there's a big unknown next to your name. Riders who have completed one or more IBA certificate rides, or who have entered one or more shorter rallies tend to be known, and therefore have a higher chance of knowing what they're getting themselves in to (although that can vary quite a bit).
Some local LD Rallies may offer an IBR slot to podium finishers. Occasionally, they might offer a fully-paid IBR entry to the winner. The Butt Lite Rally has done this in the past, which made losing all my bonus paperwork on leg 1 particularly painful, as it dropped me from 1st to 2nd place. That was a $2,000 mistake...
So my recommendations are to just ride a whole lotta miles, collect some certificates, and enter some local rallies. Follow the advice given here
, and just keep on riding until knocking out a thousand miles or more is almost boringly easy. You really need to prove to yourself that you can do it, before you try to convince others.
Or you can do what I did, which is to fail to make the draw, be bummed, but pretend like you made it and keep on prepping and riding the bike as if you were in. Then go on to win four local Rallies in a row, and wait for the phone call to offer you a slot...