You'll need to get an II account setup. Ask for the worldmark desk at II, because it's a bit of an unusual process.
Once you do that, you can request first or deposit first. With worldmark, a request first generally has higher trading power. You can make a request (or search available inventory) using the three bedroom float. When you make a trade, you are charged based on the size of the trade. 8k 1 bedroom, 10k 2 bedroom, 12k 3 bedroom or larger. (Plus housekeeping and II exchange fee).
If you deposit first, you can potentially save credits, but it's riskier. You don't get to pick the week, the transaction can't be reversed, and the deposits expire. However, you can deposit a low season 2 bedroom unit for 6k credits plus a housekeeping, which is a potentially material savings on credits. A low season week will have trading power based on the week. If you get a good one, it can be nearly as good as the float week. If you get a stinker, you might have a hard time getting something.
One other thing - during flex time (59 days before check in, IIRC ) all float trades are only 4000 credits, which can be great.