How to set signatures for a new email and an email reply?

You can easily set up and differentiate a signature template for your new emails and a signature template for your email replies.
You can access this distinction in the Signatures menu by moving the "New email"/"Reply" toggle available in the top left.
 

The settings are then configured when creating a signature: a dedicated section, called "Sending method", allows you to determine whether this signature is used when composing a new email or when replying to an email.

Once the settings have been configured, Boost My Mail will automatically select the correct signature template according to the context. Your user will not need to do anything.