Before starting, try to be in a focused and sharp mind, and have enough time ahead to dedicate to the massive cleaning. If you can, put some good music on, and get on with it.
Clean the current inbox. Delete all marketing emails (no, there is no “I’ll read it later”)
Emails that would take less than 3 minutes to answer, answer them NOW. No delay. Even if it is just to say “I will answer by next xx”.
Create folders for emails you have read and have answered, but need to keep. Start simple in the names of the folders, then add new ones. No need to start by creating a complicated folder tree as it can be improved along the way.
Unsubscribe from all emails you don’t need or don’t find relevant anymore
Other emails: start answering from most recent to oldest. Depending on how many emails are unanswered, divide the total, and just do few a day. It will always be better than none at all.
Find a time when it’s best in the day for your to answer your emails. Usually, for most of the people it is in the morning, before starting the day, and at the end of the day
Set a strategy that works for you, and stick to it. Easier said than done, but once you have seen the benefits of it, it should be easier.