The most basic form of an outline will be one in which you begin with a general idea or your thesis statement and break down that general idea into several component parts or supporting claims. Unless you have other specified guidelines to follow, you should begin the outline with your main argument and then lay out the claims that you have to make to support that argument. In other words, start with the general and move to the specific.
The structure for a basic outline is usually the most confusing part to grasp. The most common organizing system is to use Roman numerals first, then capital letters, then digits and then small letters. So you would first write out your Roman numerals, or I, II, III, IV, which would consist of the breakdown of your argument into the largest subcategories, and then you could also break down each of those subcategories of Roman numeral into smaller groups of A, B, C, D categories depending on how many you need. You will continue with these steps until you have written out the major points to your paper. The important thing to remember when performing this step is that in an outline you cannot have an A without a B or a I without a II.
There are generally two ways of writing out an outline: using sentences or using topics. When using sentences, express each component of your outline in complete sentences. When using topics, express each component of your outline in topics or phrases. You generally cannot mix the two styles in one outline; that could be confusing.
Unless you have other instructions to follow, you do not have any length requirement when it comes to a basic outline. It can be as long or as short as you need it to be, although if it exceeds a page it probably cannot be classified as a basic outline.