The first step in introducing observation to kids should be to include it as an overall lesson in the scientific method. The four steps of the scientific method are observation, formulating a hypothesis, predicting results based on the hypothesis and performing experiments to test the hypothesis. The last three steps are a bit more difficult to explain to kids; however, it is important that they at least understand observations eventually lead to predictions about how nature works.
An important lesson for students when learning scientific observation is for them to understand both these terms in their appropriate context: "science" and "observation." Many students have a sort of mental block, and think of science as something particularly hard and abstract; in reality, science is everything that we can observe. Coupled with this is observation, which is the process of noticing something in the physical world. Observation can be accomplished through myriad ways, be it simply with our senses, or by using instruments (such as a thermometer).
In order to help students understand observation, you can prepare a lesson where you have them talk about or write how they use scientific observation everyday. Any form of measurement, be it checking temperature of food or weighing themselves, is a type of scientific observation; even seeing what time it is scientific observation. Whenever they judge a physical situation, students are observing the universe around them and using evidence to make their decisions. By focusing on how they use observation as second nature everyday, students can better relate to how scientists try to understand the world.
A capstone for series of lessons on scientific observation should involve actual hands-on scientific observation in the classroom. There are several different ways to implement this. One is to introduce students to tools used in observation that they may not be as familiar with, such as a microscope or a telescope. With these instruments, they can begin to understand how scientists probe the unseen world around them. Another approach could be to set up a long-term activity and have students observe how something changes with time; for example, you can have them plant seeds and observe the plants' growth throughout the year. An important step of a hands-on activity is to make sure students are actually making observations, primarily in the form of note-taking.