The first and most important principle to be observed in constructing a paragraph is that of UNITY.Just as each sentence deals with one thought, each paragraph must deal with one topic or idea – and with no more than one. In writing an essay, for example, every head, and every sub-head, should have its own paragraph to itself. And every sentence in the paragraph must be closely connected with the main topic of the paragraph. The paragraph and every part of it must be the expression of one theme or topic.
Note: A good practice is to read a chapter in a book, and give a short heading or title to each paragraph, which will express in a word or brief phrase the subject of the paragraph