If your class has been adding whole numbers, challenge them with a realistic problem where estimating would be appropriate. For example, hand out grocery store flyers and ask students to quickly determine whether they could buy milk, eggs, flour and chocolate chips for under $20. Don't teach any estimation strategies at this point and don't labor the point of the activity. Allowing students to begin the challenge with minimal instruction promotes a problem-solving mentality, and if students experience a moment of frustration, they will be eager to discuss strategies later.
Gauge the noise level in your class, and stop the challenge while students are still engaged. For those who are frustrated that they didn't "finish," remind them that strategies -- not answers -- are being collected. Invite students to share and discuss their ideas. Some students may have computed the exact cost of the four items; praise their effort and ask them to consider if that was the quickest way. Other students may have guessed, which is very quick but not nearly as dependable. A few students may have rounded the numbers to add more efficiently, at which point you should inquire about their rounding -- did they round $4.29 to $4 or $5, and why? Some students will surprise you with other original answers. Write them all down, and invite the class to comment on each one. Responding to each suggestion will help your class understand that many strategies exist, so avoid demonstrating excitement over an answer that meets your expectations.
After attempting, sharing and critiquing various ideas, your class will be open to learning from an explicit lesson. Choose two estimation strategies to demonstrate. A simple beginner method is front end estimation -- using only the beginning digit of a number and mentally replacing the rest with zeros, effectively "rounding down." Another strategy is finding compatible numbers, where students scan the problem to pair up numbers that are close to "nice" sums like tens or hundreds. For example, when estimating 12 plus 16 plus 7, students would consider 12 plus 7 to be close to 20, and then add the 16 for an estimate of 36. Demonstrating two strategies rather than one encourages students to choose the best method for each problem.
After your brief demonstration -- ideally, 5 to 7 minutes in order to maintain attention -- students should try to implement the new strategies with realistic estimation problems. Provide a range of scenarios where over- or under-estimating would have varying impact, like when shopping, saving money, ordering school supplies, preparing food for a party or trying to reduce waste. The goal is for students to estimate in meaningful ways, not to engage in rote practice. If they ask about using a different strategy, encourage that as well.
Conclude each lesson by reflecting as a class. This provides valuable time for students to consider successes and challenges of their peers, connect new learning with previous knowledge and assess their own understanding up to that point. This is also a good time to add strategies to a "toolbox" list on the wall, and to decide on the focus of your next estimation lesson.