Go to the Inventory page. To the left side of the listed inventory, 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 inventory 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, ASAP was selected and applied to all of the currently selected Inventory Items. Small tag icons appear in the TAG column.
Note: You can add only one tag per inventory item unless it's a merged or bundled tag.
You can also add tags from inside the Inventory item. When clicking 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. The same rules apply, 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 its color. Once that is done press Save.
We can now see our New Tag in the menu.
You can also find your tags inside the 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.