Hello Sicily.
Massimo, there is no example for now but it won't be hard.
Create a new printer and it's type will be "Text Printer". "Line Character Count" will be the width of paper. It might be 80 for A4. It is important because SambaPOS calculates line alignments of lines with that value. You can fine tune it for your paper design.
Duplicate "Ticket Template" and add empty lines and spaces to fit it for your design. While testing your design you can create an additional printer and set it as demo printer. Enter a fake printer name and open Notepad application. The output will sent to notepad instead of printer.
Finally create a new print job, Enter button header as "Print Invoice". Enter a new mapping and choose your new printer and template. Open "Terminals", open default terminal and add this printjob in "printjobs" list. Now you should have a button titled as "Print Invoice" in your ticket.
Share your template and results here so it will be an example for new users
Thanks.