The following information is taken from the website of John Jay College of Criminal Justice.

BASIC OUTLINE  FORM
Below is a synopsis of the outline form. The main ideas take roman numerals. Sub-points under each main idea take capital letters and are indented. Sub-points under the capital letters, if any, take italic numbers and are further indented.
I.  MAIN IDEA
A. Subsidiary idea or supporting idea to I
B. Subsidiary idea or supporting idea to I
1. Subsidiary idea to B
2. Subsidiary idea to B
a) Subsidiary idea to 2
b) Subsidiary idea to 2
II.  MAIN IDEA
A. Subsidiary or supporting idea to II
B. Subsidiary idea to II
C. Subsidiary idea to II
III.  MAIN IDEA
It is up to the writer to decide on how many main ideas and supporting ideas adequately describe the subject.  However, if there is a I in the outline, there has to be a II; if there is an A, there has to be a B; if there is a 1, there has to be a 2, and so forth.
http://www.lib.jjay.cuny.edu/research/outlining.html