In fact, you can print directly without inserting blank pages, but first you need to insert section break (next page) at the end of each part. When printing, select "odd pages" as the printing range and "reverse order" as the printing order (check "reverse order printing" in the "Options" in the lower left corner of the printing window). After printing, put the blank pages face up in the printer, and then select "Even Pages" as the printing range. In this way, double-sided printing is realized. If the number of pages in a part is odd, when even pages are printed, a blank will be automatically printed on the back of the last page of the part.
There is another simple way. First, print all the pages on one side. After printing, add a piece of white paper at the back of odd pages, and then copy on both sides. This method can be used to print a large number of copies.