Resending an email with the plugin is very easy. Please note only, that custom comments need an additional setup step.
To resend the email you have to go to
Orders > Overview > "Any order" in the admin and scroll to the section
Resend order confirmation mail.
There click on the button
Resend mail, optionally enter a comment and click on the button
Send now to trigger the email.
As already mentioned, custom comments require an additional step, as these access the default email template
Order confirmation. An automatic installation is not possible because this email template can be very individual per store.
To customize the email template you have to navigate in the admin to
Settings > Shop > Email templates, there edit the email template
Order confirmation and then scroll to the section
Now you have to insert the dynamic placeholder
resendOrderConfirmationMailComment in the appropriate places and finally save the email template.