Find the forums that are related to your niche. You can do this by using keywords and Google operators. For example, if you need to find a forum in a 'dating niche', then search in Google as 'Dating inurl: forum'. What will happen is; this query will search for dating related sites that has 'forum' in the URL and it will take you directly to the forum page of the particular URL you select from the Google.
Find the websites that you think is suitable for you to build forum links on click on it. Note that you may also find other sites in between that would point you to other real forum meetings. Skip it.
Now read the forum rules first, generally, every forum board has different layouts and different positions. Hence, browse throughout the site. Some of the places where you can find it is 1. Inside the main forum threads as sticky posts, you'll 1st PM that you will get from Forum Admins, at the bottom of the footer, etc.,
Register yourself clicking on the 'Register' or 'Sign up' button. It will be usually at the top right or at below the header or somewhere else. Remember, forums have been one of the top backlink building sources for SEO masters and Forum boards intentionally keeps the 'Register' button at different places to avoid spam registrations.
Go to your mailbox to verify and confirm the registration. You'll get the mail to the mail address that you have entered while registration. Please remember that at times, the mail will be delayed due to some scenarios like server congestion, manual verification, etc.,
Wait for the Administrator. Even after verifying the mail, you may be put on waiting queue till Admin manually changes your membership status to 'live'. It may take from 5 minutes to indefinite time depending on the admin's availability.