In baseball, as a general rule a player doesn't declare for the draft. Every player who has just graduated from high school, or is in JC, or has completed his junior year at a 4 year school, or has turned 21 is eligible automatically for the draft. All players are eligible to attend professional tryouts.
There is a wrinkle that a 4 year college player who doesn't qualify by the above criteria can renounce his NCAA eligibility, but I doubt that's what you're interested in.
Leaving aside the rules, in baseball it is very much a case of "don't call us, we'll call you." If a player has built a sufficiently strong resume on the field, scouts will start to follow him. If he hasn't got the resume, he can attend a tryout. There's nothing else a player (or his parent) can do to "start the process."