You need to know that there is no guarantee whatsoever that your emails will be removed from the promotional tab, but below you can find some tips based on best practices:
1- When people opt-in to your list, send them a welcome email and ask them to kindly remove you from the spam or promotional tab to the main inbox.
2- Try a basic simple HTML format instead of a fully designed HTML email.
3- Rename your Unsubscribe and Preferences labels from the footer of your email for example you can use “Please remove” or “Click here to opt-out”.
4- Use proper segmentation between those who open and engage with your email and those who don’t, you should probably send them emails less frequently or suppress them from your list.