Open Gmail and sign in to your account
Click the gear icon in the top right
Select See all settings
Open the Labels tab
Scroll to the Labels section
Click Create new label
Enter a label name
Choose whether to nest the label under another label
Click Create
To apply the label to an email, open the email
Click the Labels icon
Select the label
Click Apply
To create a label from the left sidebar, click More if needed
Scroll to Labels
Click Create new label if available
Enter the label name and click Create