Go to the E-archive, click on the document, scroll down to the Parties section. Click on the party you want to send a reminder to and click on Send reminder.
You can also go to E-archive and Documents, select the document and press Send reminder.
Also notice that an automatic reminder can be set up on the Other settings tab before initiating the signing process.
Note that a signing party is not automatically reminded to sign unless you set this up under Other settings before initiating a signing process.