To add a signature to all outgoing emails - choose Options > Mail from the File menu at the top of the main Windows Live Mail screen and then select the Signatures tab

 

 

You can then click New to create a new signature and enter your required text in the box below

 

Tick the box at the top of the screen to Add signatures to all outgoing messages and choose whether you want to add signatures to replies and forwards

 

When you are done click OK

 

Now when you create a new email message the signature will appear automatically as shown below