Step 1.
Click the "Profile"

Step 2.
Click "Forum Profile Information"

Step 3.
In the Signature Section post your desired Signature.

Note:
1. Signature starts with [(img)].
2. Followed by the link of the signature.
3. Lastly ends with [(/img)]
Reminder:
Just remove the "Parenthesis"
Sample Qouted Signature:[img]http://i1222.photobucket.com/albums/dd500/lawlietoverlay/farsightsignature-0.gif[/img]
For Linking a Signature: Instruction Code:[url= (Your Facebook url here)] (your signature code)
Sample Code:
[url=https://www.facebook.com/farsight.decipher][img width=480 height=150]http://i1222.photobucket.com/albums/dd500/lawlietoverlay/Farsight-20.gif[/img]
Sample Signature:

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Credits to Farsight
Originally posted: [Guide] How to post a signature? [Update] Linking your signature.