Go to the Inventory page. To the left most side of the listed orders you will see empty checkboxes.
Click on any of them or all (I have selected all in this case). You will see how the bar above your orders has changed and now shows an additional menu.
Press on Tag to bring up a drop-down menu containing all of your tags.
In this case i chose ASAP and it applied it to all of the currently selected Inventory Items. Adding a Tag will leave a small Flag with the tags colour and when you hover over the flag it will show the tags name.
You can add only one tag per inventory item.
You can also add tags from inside Inventory item. When click on a Simple, Merged or Bundled item, it will take you inside. I've chosen the Blue Pencil in this case.
To the right side you will see a grey button titled Tag. Press on it and choose your tag. Same rules apply, one one tag can be active at any time.
Your account comes with a set of default tags, but you can add more by opening the tag drop-down again and pressing +Add Tag at the bottom of the selection.
This will bring up a pop-up window where you will be able to Name your tag and choose it's colour. Once that is done, press Save.
We can now see our New Tag in the menu.
You can also find your tags inside settings. Go to Settings -> Account and scroll to the bottom until you see Tags. Here you can see all of your tags, Edit them, Add New or Delete them entirely.