Chemistry lab reports - from the simplest high school experiment to cutting edge research in a University or Pharmaceutical company - shares certain attributes that must always be present. The degree to which the guidelines are followed varies depending on the "seriousness" of the lab atmosphere - a high school lab report is almost a joke, whereas millions of dollars could potential ride on a properly written and documented industrial lab report.
First of all, the lab report should be written in an official lab notebook. The book should be instantly recognizable, to start with. A uniform shape and size, and colored markings, make it easy to identify when searching for it in a crowded lab environment. Secondly, the notebook must be well made and robust. It will serve the practicing chemist for some months, depending on the level of his productivity, and will be expected to hold up for many years afterward - it does no good for crucial pages to fall out and go missing months down the line! The paper should be both acid / base resistant and also (to some degree) solvent resistant. A laboratory is a messy workplace and it does no good to record an observation only to have it transformed into a smudge by a stray squirt of acetone solvent when cleaning glassware.
Each page should be numbered - most already come with a preprinted number in the upper right hand page. Carbon paper copies are extremely frowned upon - don't use them. Begin each experiment with a fresh page, writing on the right side of the page only. This avoids overcrowding and "leakthrough" to the proceeding page, potentially obscuring valuable text. The following cannot be overemphasized: DATE each entry. A time stamp would also be valuable. Many times notebooks have been dragged into patent courts, and it is often to presence or lack of a date stamp that establishes precedence of a discovery; when you consider that millions of dollars in future earnings could be on the line, you begin to see the importance.
Write legibly. Begin by describing your idea. Draw a mechanism, if applicable. Then, record exactly what actions were taken. What apparatus was assembled? How much of what reagent(s) did you add? What were the "lot numbers" of the chemicals? What physical changes took place (exo / endotherms, color changes, bubbling, etc). Did you apply heat or cold to the reaction vessel? If so, how much? How long did you let the reaction continue? How did you "quench" it? How did you purify it? How do you know the material you purified is the correct material - what instrumental methods did you use? Finally, what was the yield of the desired product (how much did you make, based on how much you intended to make)?
As a final step in making a chemistry lab report entry, it is good form to leave room at the bottom of the page for a colleague - someone of technical aptitude who is reasonably expected to possess sufficient knowledge to understand the material - to sign off on the lab report, indicating that they have read and understood it. They date it, and the process is complete. At this point, any sufficiently trained scientist can duplicate your work - the hallmark of true science.