for a long time I struggled with the toss. I would get nervous in matches and my toss would be off all the time. Especially when the match was close.
I found that my fingers were the problem and when I was nervous they would mess up the toss.
I fixed it entirely by tossing with my knuckles. Like a knuckle ball pitch.
IMHO I agree with lee d. Picture where the ball will land before you serve and just make it happen. Think about the result and your body will figure it out. Focusing on all the interim parts will just mess things up.