To connect your Spark Shipping account to ShipStation, first, login here.
On the Vendor's page, click on the ShipStation tile and click Add Vendor.
Copy the username, password, and URL that appear on this page. Once this is done, go to ShipStation.
Head to your Account Settings (the wrench icon in the upper right).
Next, select Selling Channels and then Store Setup from the sidebar on the left.
On the page that appears, click +Connect a Store or Marketplace.
Select Spark Shipping from the options in the popup.
On the popup that appears, enter your Spark Shipping Username, Password, and URL.
Click Connect and you're done!