To set up your SI signature in Microsoft Outlook, do the following:
Click on File and select Options.
From the left side, select Mail.
Then, on the right side, select Signatures.
Make sure you're on the E-mail Signature tab. And, if you're creating a new signature (as opposed to editing an existing one), click New, give a name for the signature you're creating, and then click OK.
Once your signature is created (or if you're simply editing a previously existing signature), you can just click into the Edit signature large text box and start editing.
The Communications Office is asking us to standardize on one that looks like this:
St. Ignatius College Preparatory
2001 37th Avenue
San Francisco, CA 94116
(415) 731-7500 ext. YourExtensionHere
Finally, you need to select your account (if you have multiple email accounts in your Outlook client) and then select which signature will populate for New messages and for Replies/forwards.
If you use Apple Mail for your e-mail, there are instructions here.
If you use Gmail's web interface for your e-mail, there are instructions here.