Use Multi-level List
First, it's a good practice that you finish writing your list completely and continuously without numbering and without text lines that are not list items. This should avoid converting numbers to default options which may not work well to give what you expect.
Highlight all your un-numbered lists and from the Home tab on the Ribbon, click the arrow with the Multi-level list button (grouped under Paragraph, 3rd from left). From the context menu it has, pick Define new multi-level list
. And before you change anything from the dialogue box, click the More >>
button at the bottom.
From Click level to modify
at the top-left, pick 1
.
Below it at Enter formatting for number
space, clear it and and write Part
(with space).
Below it at Number style for this level
, select a numbering system. Your done with this level.
Go back at Click level to modify
at the top-left and pick 2
.
At Enter formatting for number
space, clear it.
At Number style for this level
, select a numbering system.
At Enter formatting for number
space, put the cursor at the end (Keyboard END) and type in a fullstop .
.
On the right, un-tick Restart list after
. Your done with this level so confirm your changes.
Now highlight the list items that should lie under Part 1, for example.
Click the arrow with the Multi-level list button (grouped under Paragraph, 3rd from left). From the context menu it has, pick Change list level
and pick the second level. Repeat it with the other sub-items in you list.
If you want to insert normal text lines between 2 list items, put the cursor at the end of the upper item list. Press Keyboard ENTER 3 times till the cursor moves to the beginning of the new line. This will maintain correct numbering through all your document.