Open Gmail in a web browser or the Gmail app
Sign in to your Google account
Go to the Inbox or use the search bar to find messages
Select the checkbox at the top to select all emails on the current page
Click the option to select all conversations in the inbox if it appears
Open each email to mark it as read
Use the More menu and choose Mark as read for selected emails
Search with `is:unread` to show only unread emails
Select all unread emails and mark them as read
Check other folders such as Promotions, Social, Updates, Spam, and Trash
Use the All Mail label to view every email except those in Trash and Spam
Repeat the process until no unread emails remain