Connecting your selling channel(s) to ShipStation is a simple process! If you have multiple selling channels, that's totally fine. ShipStation does not limit the number of selling channels you can add! Do keep in mind, different selling channels may require slightly different steps, but ShipStation provides step by step instructions through the entire process.
This short video illustrates the basic process of adding a selling channel. More detailed steps included below:
Add a Selling Channel to your ShipStation Account
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. 
Now, you'll see a list of the stores currently attached to your account, if any. To add a new one, just click the Connect a Store or Marketplace button.
Choose the platform or channel upon which your store resides, and then follow the instructions on that screen to complete the process.
Once the store is connected, you'll see your Store Setup window. Here, you can set the name of the store as it will display in ShipStation. You can also customize any of the other store options within each tab. For example, if you'd like to setup your Branding options so your store name and logo can be included on your confirmation emails and packing slips, go to the Branding tab, as shown in this image:
If you are not ready to make these changes, don't worry! You can edit them at any time in your ShipStation Store Setup screen.
When you're done, make sure to click the Save Changes button along the bottom of the page.
Congratulations! You are now ready to start importing orders from your selling channels!