First: make 30 or more contributory posts in the forum in a given calendar month.
I try to make between 40-50 each month - or 1 or 2 a day.
Towards the end of a given month after you have more than 30+ posts, and when you are ready to request the bonus, click on the "Paid to Post" link at the top of any Forum page. (Feel free to check this page out right now to get a feel for what it's like as well as the information shown.)
This will take you to the page that describes this bonus. In the middle of that page is a green link to the "forum bonus request form". There is also a description of a contributory post right above the green link.
On the right side of Forum Bonus Request Form page is a link to allow you to view all of your posts. That's important because you need to select 3 of your posts (the ones you like best), that you want to put in the Request Form
At the top of each post, you will find a Message ID number. The posts are listed in reverse date order. Select the 3 posts you like, write down their Message Id numbers. Make sure all of the dates are for the current month.
Go back to the Posting Bonus Request Form and enter the 3 message ID # (one on each line). (If you hadn't submitted a bonus request form for the previous month, you can also do that too -- but make sure the dates of the posts correspond to the date in the drop down menu.)
Check off that you agree to wait 65 days and then click Request Bonus.
The Bonus is usually credited towards the end of the month following the month you made the request. (The bonus for January postings requested on January 31st will generally be credited around Feb 28th +/- a couple of days. The bonus for January requested in Feb, however, will not be credited until March 31st +/- a few days. (hence the potential for up to 65 days).
If you scroll down the Bonus Request Page, you will see a section labeled History. As each of your bonus requests are submitted, they will show up there -- labeled "Pending" until "Approved/Paid" or "Denied"
Posts made to the Contests and Promotions section of the Forum do not count toward your 30 contributory posts. (Probably because you already get bonuses for posting there). Also posts in the Word Chains and Countdowns don't count.