With the launch of Salesforce's Process Builder, you can easily setup automation to merge documents when certain actions happen in Salesforce.
For example, you can automatically generate documents when new records are added to Salesforce such as a new client or when certain fields are updated within a record.
To setup the Process Builder for WebMerge, go ahead and create a new process. We're going to choose the Object that we want to monitor, then we're going to add an Action. For the Action, we're going to choose Apex as the "type" then pick "Generate Document" as the Apex Class.
For the Salesforce Record ID, we're going to choose a "Reference" to our object ID. Then for the WebMerge Mapping ID, we're going to enter the Salesforce ID for our Mapping (you can find this URL by going to the Edit Mapping page and looking in the URL of the page for "Id=...").
Here's what all those settings look like: