Never used that term and I assume you mean "contribution rewards"?
It's easy:
> post GB attachment
> post the positions you want taken. EG: "1st + 2nd available"
Someone else:
> Posts the position they're taking. EG: "1st"
> It's not necessary to wait for the previous players spots to be taken before posting your own. If multiple GBs have spots still available to claim then also state the owners name that you are claiming for. If a GB hasn't had its stated spots taken and appears to be getting buried it's courteous to mention the GB and owner still has a spot available.
> they should close and reopen the thread so it reloads the order of messages. If cross-posting occurs then whoevers message went through first receives that spot.
> they then place the amount on the GB
It's also important that everyone in the guild know the rules of the thread. Is it the owner or the contributors responsibility to secure the position? A 1.9 thread would need spots secured by the owner before posting. A any amount type thread would be upto the contributor to secure and they'd need to decide if that position is worth it before claiming the spot
It should look something like this. For a smaller guild where there's less activity it may be necessary to state the current lvl of your GB to prevent confusion after it's lvl'd:
Also works best if the guild has a "no unsolicited contributions" policy. If the owner hasn't posted the GB and stated the position available then don't put anything in