Signatures contain information that you want to include at the bottom of all of your posts. This might include pictures, quotes, etc. These are perks of a paid membership supporting the site. Support the site and get more perks in the process!
To do this, look for your username in the upper right-hand corner of every page. Click on it, then click Dashboard.
On the next screen, you will find the following options: (click on it)
Scroll and click the SIGNATURE option, follow the directions, then click SAVE SIGNATURE.
If you are on mobile and want to make a change, look for ACCOUNT link.