Sometimes the purpose can be stated in one sentence. Other times it may be necessary to add some extra information to narrow the scope of the activity.  Be sure to include any necessary background information needed to explain the reasons for doing the activity.

Include a brief description of the procedures used in the activity.  Technical writing is very "cut and dry." All you are trying to convey is a mental picture of what you did. Ordinal phrases are not necessary. The order of events is conveyed by the sentence order in the description.  Emotions (This was hard. or This was fun.) are not necessary and detract from the purpose of this section. 

Use the passive voice, not the first person active voice.  (ex. "Six petri dishes were filled with agar."  instead of "I filled six petri dishes with agar.")  Describe only the procedures you used, if something was altered from the original written procedure be sure to describe the changes, not the original procedure.

Data:

This section should include only those things that you saw, heard, touched, or smelled. This includes both quantitative (numerical) and qualitative (sensual, not emotional) observations. Quantitative observations are best presented in data tables. Units are necessary for any measurement. Qualitative observations may be organized in table form or paragraph form. 
 

Analysis:

This is the section where you will show any calculations that you made using the data you collected.  You will also answer any analysis questions that go along with the lab.  Be sure to answer in complete sentences.  You may need to refer to the background information provided on the lab sheet, your textbook, or class notes.

Conclusion:

Include a brief restatement of the purpose and the main results and how they are relevant to the field of study.  The conclusion should clearly tie the results of the experiment to the hypothesis and a discussion of why the hypothesis should be accepted or rejected must be completed in detail.

Explain any uncertainties in the observations/measurements.
Identify and explain how sources of error influence the lab results.   (some sources of error could include equipment error, procedural setup error, human error,  etc.)