The purpose of an outline is to create a skeleton, more or less, of the most important ideas contained in the reading material. History textbooks usually facilitate the outlining process fairly well in that they are structured into chapters, sections, and sub-sections of information. For example, a high school textbook might have a chapter on how the nation tried to avoid the American Civil War, sections on the compromises reached in the years prior to the war, and sub-sections on the major elements of the compromises. An outline for the first section might thus appear:
I. Causes of the Civil War
A. Missouri Compromise of 1820
1. Slavery outlawed north of southern border of Missouri
2. Missouri admitted as slave state, only exception to 1
3. Fugitive Slave Act will protect Southerners
If you follow the ideas as they are presented on the page, you should be able to create an outline fairly easily based on the structure of the text.