Now, I'm looking for where to add a signature.

Menu -> Mail accounts -> select the account -> edit -> switch to Outgoing tab -> Edit Signature".

If you're going to create an HTML signature and you want to add a picture to it with <img src="">, don't add the img tag in the signature editor or it'll crash Opera Mail. Instead, goto "Menu -> help -> About Opera Mail" and take note of the mail directory location. Then, close Opera Mail and edit the signatureN.txt file in the mail folder where N is the number for the account as seen in accounts.ini. Doing things that way will avoid the crash. If you're not inserting an image, no need to do all of that.