To add a Label to Cards on a Board, you need to first make a label:
1. Open the Board and press the three dots in the top right corner of the board
2. Select "Labels" from the dropdown menu, which will open the Label Management window.
3. Click "Add a new label" to create a label, you can make as many as you like. Hit "Save" when you are finished.
Once you have created your labels, you can add them to items on the Board:
1. Press the cirlce in the top right corner of the item to select it.
2. Press the label icon from the bulk actions menu that appears
3. This opens a list of all your existing labels on this Board. Assign labels to items by ticking the checkbox in the "Add" column next to that label, you can select as many as you want. You can also remove labels by ticking the checkbox in the "Remove" column. Hit "Save" when you are finished.