To connect your Shopify store to your ShipStation account, just follow the steps below:
First, go to your Shopify store and login. Once you've done that, copy the first part of the URL of your dashboard (before the "/admin")
Log into ShipStation to start the connection from ShipStation's side.
Head to your Account Settings (the gear icon in the upper right).
Next, select Selling Channels and then Store Setup from the sidebar on the left.
Then, click on the Connect a Store or Marketplace button.
Enter in your Shopify domain that you copied previously from Shopify and click Test Connection.
Once you do that, you'll be redirected to your Shopify site. You still should be logged in from step #1, but if you're not, go ahead and login now.
Once the app installs (which should take only a few minutes at most), you'll be redirected back to your ShipStation account, to set up your Store information, Customer Communication preferences, product handling, and service mappings. If you were already logged in, you'll go directly here after hitting Finish! Save Changes and your orders will begin to import!