Import years of sales statistic from Visma Mamut into Odoo v12

Looking for someone who's great at import and export in Odoo. We started working in Odoo in June last year and we wanted to import out statistics of sales into the system to make it easier to compare sales between the years. This never happened and we don't really know how to do it. I am talking about approximately 300-400 orders a month and 2-3 rows per order. We want to import the invoices into the accounting part of the system to be able to compare the different product categories in an easy way between the years. I am thinking three years of statistics. What I want you to do is to give me give different solutions on how to do it, sort the excelfiles (should be quite easy) and tell me how to import it. We got a test-database.