Open Gmail and sign in to your account
Click the gear icon and select See all settings
Go to the Labels tab
Scroll to the Labels section
Click Create new label
Enter the folder name
Click Create
To add emails to the folder, select the email
Click the Labels icon
Choose the folder name
Click Apply
To move an email into the folder, open the email
Click the Move to icon
Select the folder name
To create a subfolder, click Create new label
Check Nest label under
Select the parent folder
Click Create