Have you set up all the email settings like an email server/port already?
If so, create a microflow that has the User entity as an input argument. From there you can call the 'SendEmailMessage' microflow from the Email module. Fill in all the arguments and it will send the mail to that user.
You can also create templates and use those with the 'SendEmailUsingTemplateName' or 'SendEmailWithSelectTemplate' microflows.
In case you want to mail multiple users at once, just loop over the users you want to mail and call your previously created microflow for each of them.