For example, if you have a product on automobile Paint Protection, you can participate in forums that talk about cars, motorcycles or boats. The owners of these vehicles would certainly be interested in the paint protection product that you have to offer.
However, you don't blatantly advertise your products on these forums on every post in a very hard sell manner.
In forum marketing, you want to give values back to the community and at the same time, insert a small ad that subtly brings the forum readers to your website. It is done by leaving a short description of your product in your signature file together with the link to your website.
Whenever you ask, answer or leave comments on these forums, it's like you're participating in the discussion and trying to help the community to learn more about the topic they are discussing. If you give enough value to the community and the readers trust you, they'll want to check out what business you have and what product you've got to offer. The signature file will do that and it appears on every forum post that you make. Since you are a trusted person, the readers will be more open to buy products from you.
As a quick tip, you also want to craft your signature file well so that when forum readers read your signature file, they'll be interested and curious to see what's of your website. This is when you write a short description of your product that spells out benefits to the readers or write something that will make people wonder that your product is all about.
However, I see some signature files are just plain simple website URL but enough to get people to check out the website. So, this is something that you can experiment yourself.
In short, this is the very basic of forum marketing with a signature file. Of course, there are lots of ways that people do in forum marketing, including spamming. I don't recommend or encourage you to do spamming on forums, if you asked me. It's because I believe that forums are more than a place to do advertising. It's also a place to share ideas, mingle around with like-minded people and network.
Pick a few online forums that are appropriate for your product promotion, create a signature file and participate in discussions. I'm sure you'll be more creative in doing forum marketing afterwards.