If you wish to appoint tags to an applicant, it is so easy. Just go to the Talent Pool and you will notice a column named Tags. To add tags, you need to click on the plus sign that is visible at the tags column of your desired applicant.

After clicking on the plus sign, type the tags you wish to add or select the tags from the list which consist of the tags added from Settings. If you type a tag that is not on the list, then you should click on Create or press Enter.

Click on Assign tags and you will notice the tags will be appointed to the applicant.