The steps for configuring signatures for Webmail differs per the Webmail client you're wanting to use.

Roundcube
Step 1: Click on "Settings" at the top right of the client.
Step 2: From there, click on the "Identities" tab.
Step 3: Double click on the email address that you'd like to configure the signature for.
Step 4: Configure and save as necessary.

SquirrelMail
Step 1: Click on "Options" towards the top of the client.
Step 2: Click on "Personal Information".
Step 3: From there, you may configure your signature and save.

Horde
Step 1: Click on the options icon at the top of the client.
Step 2: Click on "Personal Information".
Step 3: Select the default identity from the popup, configure your signature and save.