Combined Montly Billing For sales order generated in calendar month

I required to do monthly billing for all the sales order generated in calendar month e.g. 1st Jan 18 to 31st Jan 18.

I need to give consolidated bill for all sales order generated in that calendar month.

In current system when I choose to get items from multiple sales order multiple rows created for each sales order, I want to sum up items and add only one row corresponding to each item.