Johnson started his project by marking qotations in literature to show the various uses of words. Contrary to other dictionaries, he did not start with a list of words because he assumed this list would soon appear by itself. It was a natural result of the illustrations he collected. In the end, he would simply have to compare it with other dictionaries to see if the list was complete.
The work really started when the contract with the booksellers was signed and Stewart was hired. Johnson worked his way through large amounts of books but limited his field of research to the period between Sir Philip Sidney and the Restoration. He did so because he thought that later examples of language were influenced too much by French and earlier ones were simply to rude and rough. Although he took these boundaries seriously, Johnson quite often incorporated authors older then Sidney and younger than the Restoration. Examples are Pope, Swift, Abruthnot, Wiliam Law, Edward Young and James Thomas. Sometimes he would even quote living authors. Modest that he was he quoted himself only anonymously.
Johnson´s sources could not be limited to any special field. He quoted poetry, prose, works of theology, works of law, art history, history, politics and even technical works.
In the beginning he had planned to include quotations to educate the reader. He did not only focus on clarity of illustration but also on educational value. The underestimated workload forced him to abandon this aim. Nevertheless he continuously tried not to transport ideas he did not support. For that reason Hobbs is not quoted once although Johnson thought him to be an authority of language (Reddick 34). Writers were not only excluded because of their political views but also for their religious beliefs. Samuel Clarke was well admired by Johnson for his writing, but he did not quote him once because Clarke denied the Holy Trinity.
One constant source of criticism on Johnson´s work is the fact that he took all his examples from written work, therefore ignoring spoken language entirely. This criticism he accepted as correct and defended himself that it would be too much work to collect words and quotations from spoken language. It was easier to concentrate on written words. Nevertheless it was not that easy. One of the problems that made his work harder was the fact that Johnson had only access to a limited number of books. His library contained a large number of books and he could borrow some more books, but still the number seemed rather small. Johnson still wanted to write a dictionary that contained the entity of English Language. To extend his corpus, he wrote letters to friends to ask for books. The problem was that he returned them full of his marks and many owners were not amused about the way their expensive folios were returned to them. Others were more or less happy to receive their books back full of marks and treated them as some kind of curiosity. For most of Johnson´s contemporaries his use of books was very strange for they were generally seen as a symbol of status.
Johnson simply used books as tools. He marked interesting passages and did not care too much if those marks could be erased or not. Johnson´s process of selecting important passages is still some kind of a mystery. Even contemporary biographers described the process as unsteady. All books he used and that are preserved until today emphasize this picture. Sometimes he marks some pages than he leaves some hundred pages untouched to proceed with the next five pages. Johnson seemed to have picked pages at random, marked them, left the book behind and returned to the book hours or days later on another random page.
This way of working seems to correspond with his way of reading. The poet Mary Kowles said that he often did not finished books when they bored him and that he often would read only passages.