I fished as a co-angler in the BFL's, Bass Open's, and on the FLW Tour. It is a great way to learn new techniques and ways of approaching a day's fishing. What makes co-angling frustrating is also what makes is such a great way to learn. By fishing with someone unfamiliar, you will be put into situations where you normally would not fish, and it forces you to adapt your presentation or simply not catch any fish.
The way everything works is each tournament will have a "Priority Deadline." What that means is if you have a person fishing as a boater, and you sign up together, the tournament director will know that there will be a boat for the co-angler, and you guys will be considered each others "guarantee." If you sign up with a boater, and do so before the priority deadline, you will be given priority to fish the tournament over co-anglers who did not sign up with a "guaranteed" partner.
But, even if you don't have a guarantee, you can still very easily get into the tournament depending on how quickly you registered. You can call FLW and they will tell you if you are confirmed for the tournament (you are in) or you are on a waiting listing (not yet in) and they will even tell you where on the list you are. Even if you don't get a confirmation before the tournament, if it is close, I would still go because there are times when they will need an extra co-angler and will take anyone from the crowd who is looking to fish. In fact, my wife was able to fish a tournament because they were looking for more co-anglers.
For assigning partners, they do that at the meeting the night before the tournament. They will call a boat #, then the boaters name and then the co-anglers name. Also, an area will be designated for meeting up with your partner, so you walk over there and start looking for the guy who has the same boat number as you. You guys swap info, figure out a meeting time/place and then head back to the hotel to get ready for fishing the next day. Be sure to ask your boater how he plans on fishing, and what type of structure/cover may be around, so you can be sure you are as ready as can be. But, take it with a grain of salt because a lot of times the game plan will change an hour or two into the day. Just remember to stay calm and be flexible with your fishing. Most BFL's that I co-angled in, if you can catch 3 decent keepers, you are going to be close to getting a check, and 4 decent fish will almost guarantee one. 5 fish, and you will have a good shot of being in the top 20. That is not always the case, and each lake will fish really well some days, and really tough on others, but in the BFL's, when I was able to put 3 in the boat, and still have time to fish, I always felt really good about my chances of riding home on a check.
Good luck, and have a lot of fun!