To add a signature to all outgoing emails - click File > Options and then choose the Mail section - from here you can click the Signatures... button (as shown below) to create/modify your signature settings

 

 

You can then click New to create a new signature - you will be asked to choose a name for this signature

 

You can then enter your signature text in the box below and format it as required

 

Now choose which account you wish to use this signature with, and select whether you want to add signatures automatically to new messages and replies/forwards

 

When you are done click OK to save the changes

 

 

Now when you create a new email message the signature will appear automatically as shown below - or if you have chosen not to add it automatically you can click the Signature button (as highlighted below) to insert your chosen signature