You would need to have extensive experience to become an estimator. Contractors will not take on paper trained estimators. they want to see proven records of past estimation positions. You need to be able to look at a prospective contract, be it by drawings or on an existing site and see where the problems /money loser areas are and cost appropriately. There business is at stake!!
It's not as easy as many people think it is, and why you normally find the majority of company estimators are middle aged and upwards.... They know what they are looking at!! lol!!!