Spend some class time introducing the students to the concept of estimation. It's important that you define the skill before expecting the students to be able to practice using it. After explaining what it is, ask the students to brainstorm some ways they could use estimation in everyday life. If students have already used estimation to help them solve a problem, ask them to contribute the story of that experience. And if the students are struggling to come up with examples for discussion, volunteer your own easy-to-grasp examples.
Choose a selection of fairy tales to read to your class, and then pose questions to the students regarding how estimation could be applied to certain parts of the story. For instance, read "Jack and the Beanstalk" to have them estimate how many beans Jack would have received if he'd been given an entire handful, rather than just a few. Have the students test their theories by counting a handful of dried beans or jellybeans. You could conduct a similar exercise with "Hansel and Gretel" by asking students to estimate about how many breadcrumbs the children were able to get out of their supply.
To help students practice their estimation skills, host a series of contests to see who is able to make the most accurate estimations. For instance, count out enough candies to fill a jar and make a note of the grand total; ask the students to write down their guesses as to how many candies the jar contains, and then award the jar to the student whose guess comes closest to the actual number. Another idea for an estimation contest could be to bring in a piece of fabric or wrapping paper with repeating images; count out the exact number beforehand, and then ask the students to submit guesses to determine whose estimation is closest.
For a tasty, educational treat at snack time, parade the students down to the cafeteria kitchen for an exercise in estimation and how it can be applied to cooking. Choose a simple recipe that calls for an imprecise measurement, like a pinch or a dash of something; something like trail mix would work well for this kind of activity. Have the students estimate just how much of the ingredient to put into the mixture. Discuss your results while you eat the treat as a class.