The first way

Right-click the relevant contact, and select Tag from the drop-down menu.

The second way

Select the contacts by ticking their respective checkboxes, then click on the Action bar.

The third way

Drag and drop contacts to a specific tag in the Tags list in the left pane to open the Add/Remove Tag form.

In this form, you can add a new tag, remove or reassign a tag to a contact.

Tip

To add a new tag quickly, click Add in the Tag pane.