First, you need first hand experience of the trade skills used to produce the product you estimate.
Second, and education in cost accounting is extremely helpful.
Third, you should have a working knowledge of the computer aided design software common in your industry. Product specifications and drawings are increasingly sent electronically. You need to be prepared to review drawings, reply with alternatives, and provide concepts for production equipment and tooling.
It will be in the interest of your customer and the sales people to keep estimated costs low. It will be in the interest of your employer to get a high price. The estimator is in between. On a scale of one to ten, the stress level an estimator is a ten -- right up there with police officers.