Use a pencil and paper to draw an outline for your answer, if allowed. This is especially helpful for estimation questions that require extensive mathematical consideration. Use the outline to record any numerical values required to answer the question. This will ensure that you do not forget the numbers later. Visually outlining your answer on paper also helps an interviewer see the depth of your problem-solving process.
Consider what type of answer the asker seeks. In informal settings, estimation questions may adequately be answered by an unspecific estimate. However, in academic and career scenarios, estimation questions generally require you to give a specific estimate. The point of the estimation scenario is to gauge your ability to formulate accurate answers through mental consideration, as well as to display the thought process you utilize when problem solving.
Show your full thought process as you answer the estimation question. Don't assume the asker will make the same mental connections as you do. Rather, include each consideration you make in the answer. For example, if the question asks you to estimate the number of bricks in the exterior of the average house, you have to consider the size of the average brick, as well as the size and shape of the average house. Articulate all of these processes so that the asker sees the thoroughness behind your estimation.
Make your final estimate to answer the question. Don't be afraid to be too specific, as this shows confidence in your answer. In fact, most formal estimation questions require a specific answer. The process that leads you to your answer is just as important as your final answer. If the method and conclusions of your process are thorough and accurate, the answer you arrive at will be a suitable estimate.