Give your students a sample situation in which estimation would be more helpful than careful calculation. For example, tell them to imagine they are at the store and they want to buy six snacks. Each one costs $3.99.
Tell the students that they only have $20, and they do not have a pencil and paper or a calculator. Ask them how they can figure out whether they have enough money for all six snacks.
Point out to them that $3.99 is very close to $4. Ask them whether they could solve the problem easily if the snacks cost $4 each. When they say yes, tell them that this is the purpose of estimation. On a multiplication test, they would need an exact answer, but at the store, estimating is good enough -- and much easier.
Fill a jar with beans or other small objects, and display it at the front of the classroom for a week. Give students the week to estimate how many beans are in the jar. Have them write their guesses on slips of paper and deposit them in an envelope by the jar.
Read the guesses out loud at the end of the week. Tell the students what the correct number was, and give a small prize to the student whose estimation was closest. Assuming that the winner did not guess the exact number of beans, point out that the prize was not for guessing perfectly but for guessing well enough.