This is done for your whole account in your gmail account settings.
Acessing your account with any device will use signatures once they are configured.
As you will see, it is quite simple. This is ripped off of google signature help page
How to create a signature
Click the gear icon in the upper right, then select Settings. Enter
your new signature text in the box at the bottom of the page next to
the Signature option.
Click Save Changes.
Signatures are separated from the rest of your message by two dashes.
To see a signature in Gmail, click the Show trimmed content button at
the bottom of the message.
If you notice extra characters (such as * or >) being added to your
signature after you send mail, it's because some versions of Gmail
don't support rich text and we've converted some of your formatting.
For example, when you send from Gmail using Safari on iPhone, any text
you bolded in your signature will be marked with asterisks (i.e.,
example will show as example).
For rich text signatures, the maximum amount of characters you can use
is 10,000 (including HTML markup).
If you send mail "from" multiple addresses in Gmail, you can set a
different signature for each address in the General tab of your
settings. Choose the second radio button in the "Signature:" section,
use the drop-down menu to choose the appropriate address and set the
signature you want.
If you're editing your signature and only have an option to create a
plain text signature, this is due to your current compose message
settings. Click Compose to create a new message, then click the Rich
formatting option in the message. Once this change is made, you'll be
able to create a rich text signature.
Hope this helps, cheers!