Depending on the size of the spray bottle you are using, add equal parts of white vinegar and clean water in it. Be sure to note the expiry date of the vinegar. You would find it on the bottle label.
Do not use expired vinegar.
Vigorously shake the spray bottle in order to obtain a proper mixture of vinegar and water.
Spray vinegar-water mixture from the spray bottle onto the mirror you want to clean. Make sure that the entire surface is covered with the cleaning mixture.
Use a piece of clean soft cloth or a paper towel to wipe clean the mirror surface in a circular motion. You might have to use more than one paper towels or a new piece of cloth. Continue until all the accumulated dirt on the mirror surface has been removed. When you are done with this step, the mirror you just cleaned would be shinning like new.
You may have to repeat the previous step more than once because heavy accumulation of dirt on the mirror surface is difficult to get rid off. Same will be the case if you are cleaning grease or oil stains from the mirror. Irrespective of the number of times you may have to repeat the third step, be sure to use a clean piece of cloth or paper towel at all times.