What you mean, I think, is how to WRITE an abstract. It really isn't all that hard. An abstract is simply a brief summary of your essay or project. 5-6 sentences are usually adequate. Look for your main ideas, what you did, and your conclusion.
An abstract must be a fully self-contained, capsule description of the paper. It can't assume (or attempt to provoke) the reader into flipping through looking for an explanation of what is meant by some vague statement. It must make sense all by itself. The actual process of writing an abstract will force you to justify and clearly state your aims, to show how your methodology fits the aims, to highlight the major findings and to determine the significance of what you have done. The beauty of it is that you can talk about this in very short paragraphs and see if the whole works. But when you do all of these things in separate chapters you can easily lose the thread or not make it explicit enough.
An abstract is a self-contained, short, and powerful statement that describes a larger work. Components vary according to discipline. An abstract of a social science or scientific work may contain the scope, purpose, results, and contents of the work. An abstract of a humanities work may contain the thesis, background, and conclusion of the larger work. An abstract is not a review, nor does it evaluate the work being abstracted. While it contains key words found in the larger work, the abstract is an original document rather than an excerpted passage. You may write an abstract for various reasons. The two most important are selection and indexing. Abstracts allow readers who may be interested in a longer work to quickly decide whether it is worth their time to read it. Also, many online databases use abstracts to index larger works. Therefore, abstracts should contain keywords and phrases that allow for easy searching.