How you go on this is kind of dependent on how close to licensure you are, and your long term goals. If you're close to licensure you might want to get your ticket punched first.
As an estimator you calculate the materials required to perform a job, and how many man and equipment hours it will take to install those components. That gives you the bare cost, and then you add overhead and profit onto that determine the estimate or bid price for the project. The estimator makes or breaks the company. They get it right the company makes money and grows. They get it wrong and the company dies.
Pay varies by company, and the estimator's skill. As the junior you would probably be on a salary. It will probably be about ten or fifteen percent above what you are making with your bags on. But long hours when you are getting ready to put in a bid are the norm, plan on a fifty plus hour week. As you learn and start bidding jobs on your own the pay gets better. Best case scenario: You get to the point where you get salary, plus a percentage of profit on the jobs you bid, and/or a part of the company.
Top notch estimators are critical to a companies success. They can almost always call the hall and get journeymen, and masters. Top notch estimators are rare.