To start, go to Sales > Orders
Table of Contents:
1. Click on the corresponding row of the default label column (or a custom column you've already created) from the Order Table. If not already labeled, you should see a faint flag symbol in the column (pictured below).
2. From here, you can choose to assign a label you've already created. You can also choose to un-assign a label that has already been assigned to an order.