How to make invoice based on a template?

I am a new computer user regarding office packages like Libreoffice. I have no training regarding using Libreoffice or MS Office. I just want to improve my skills and make better use of all the features in the software.

I prepare invoices, what I do currently is copy the blank to a new sheet and update the info.

Is there any better way of doing it? I would like to just put the info of the invoice in a table on 1 sheet (this make it easy to look at all the info instead of going sheet by sheet, currently I have an extra sheet just for that) and have that reflected to the main sheet.

I heard of feature called mail merge and saw a tutorial to do in Excel. I saw an old one for Libreoffice too. But that is connecting external datasource.

Can I just connect to the sheet in the same file? Will mail merge get me what I want? I want to just have one sheet for the invoice and I can choose the invoice number via drop down.

If not mail merge, what is the right way?