This is probably easier than you think. The roman-numerals (I, II, and maybe III) are your biggest ideas. You write those into the outline first. After that, you break each down into smaller ideas, these ideas are A, B, C, and so on. Then you break those ideas down even more into numbers (1., 2.).
Here's a brief illustration
Thesis: The city is a great place to live
I. The city is great because there are lots of things to do
A. There are amazing shows
1. Blue Man Group
2. Broadway Musicals
II. The city is great because there are great foods