You write a dissertation on a topic that has your advisor's approval. Depending on the discipline you are working in, your dissertation might be about laboratory experimentation, some new view of earlier writing and thinking or something else. Your advisor reads each chapter as it is written and when the paper is finished, copies are sent to the members of the committee that has agreed to read and approve or disapprove your dissertation.
Normally, you have to do a review of previous work that is related to your work. If your paper takes a very unusual slant, it may be difficult to find earlier work related to your own but you do your best. What you did yourself in the laboratory and the library and wherever else you carried out your research need to be described. A section explaining what you have found will give the results of your work. Finally, you would have a last chapter of conclusions that your work leads you to and implications of your work for the future.
Depending on one's personality and luck, the dissertation committee may be rather favorably impressed with what you have written or not. The committee is aware that rejection of your paper would be painful but on the other hand, acceptance of shoddy or shallow work reflects badly on the institution. The discussion of what you have written will probably last for about an hour or so. Then, you will be asked to step outside while the committee discusses in private. If you pass, when the committee member steps out to ask you to come back in, that person will often say "Dr….., please come in."
There may be some corrections and adjustments to the dissertation before it is accepted but if you pass, you will be on your way to the next step in your life.
