As Cmays says there are differences with a 1 or 2 plane swing here. But for a 1 plane:
If you imagine a line traced from the target that passes through the ball at address and out behind you. Now imagine a laser light that is shining out from the butt end of your club.
As you complete your takeaway the light will now trace this line right to the top. As you come back down the light will again trace the line to around the release position. As you extend past impact the light will again return to the line in front of you and track towards the target.
Butch Harmon has a training device that does exactly this:
Golf Laser: Butch Harmon Golf Laser Trainer
For a two plane swing you want the butt end pointing at a line a little inside the line when halfway back and again at release.