1. Go to "Messages" --> Messages templates"
2. Choose "Create template" in the top right corner
3. In the top, you can choose if the mails should be send by a trigger as well as a meeting invitation (doesn't have to send by a trigger, if nothing is chosen, the phoner can manually send it):
Afterwards you need to choose the name of sender, email of sender, email of recipient, BCC (Not required) and Subject. When creating a template, you can use "merge tags" which are found in the right. Merge tags help you customize the mail according to the info you have on your users as well as the lead.
An example could be that you want the Phoner who called the lead as the sender. Here you can use the merge tag at the right side.
The merge tags can also be used in the template, for an example if you email should start with "Dear (customer name)" merge tags for the customer name can be found on the right.
Please note that the merge tag for lead master data, result data is different depending in your specific fields.
4. Afterwards you can save the template.