1) Before you'll be able to connect your store, you will need to install the extension into your Magento Community store. To do that, you'll need to make sure that you're logged into your Magento store. Then, go to the ShipStation extension page in Magento Connect and click Install Now. (If you need help with installing the extension, please see this article.)
2) Once that finishes, you'll need to now create a user for the app to use when connecting the two systems. To do this, go to System, Web Services, and then choose SOAP/XML-RPC - Users.
3) Now, click on the Add New User button.
4) Here you'll need to fill in the information for your new user. Make sure to enter "shipstation" as the username, and you will need to create your own API key. Usually, these are 40-character strings that includes both letters and numbers, though you can make them whatever you wish. Make sure to enter the same thing in both the API key and the API Key Confirmation fields. Also, make sure to copy the API key before you leave this screen.
5) Once you've done that, simply click on the Save User button.
6) Head to your Account Settings (the gear icon in the upper right).
7) Next, select Selling Channels and then Store Setup from the sidebar on the left.
8) Then, click on the Connect a Store or Marketplace button and select "Magento".
9) Here, enter the username, API key you copied, and the URL for your store and then test the connection. (If you are using a Magento Multi-Store set up, you'll need a special store URL for each Magento store you add.)
10) If you use any custom statuses, you'll need to change them while you're setting up your store in ShipStation. Also, at this point you can add your Warehouse Location Attribute and select when Magento will invoice.
When you're done, click Finish to fill in the rest of the information for the store.