There are many ways to aim for a spot on a dartboard. The following is not the only definitive way to do it, but it is one way you can try until you find your most comfortable method:
Find the spot you wish to hit and draw an imaginary line that extends from the board, through your hand, and in front of you. Place your dominant eye in a straight line with the dart and its target. When throwing, make sure everything stays in a straight line:
From the aiming point on the dartboard through to where your arm is extended in front of you
From the arm extended with elbow locked through to where your wrist is pointing straight down toward the ground
To find out which is your dominant eye, you can refer here
Check out the video above if you want to know more!