I need help throwing a sinker? Any suggestions?

I'm exactly how you are..same age and throw the same velocity. I asked about sinkers and I know it's a 2 seam but thrown differently. What people have told me is to rotate your arm the opposite way of the way you would throw a slider. To help they said dip your thumb to your left hand side. That's all I got.

I was at Academy Sports and Outdoors, and I picked up one of those balls that have dots for finger placement for certain pitches, looking to see if it was indeed correct, and a sales guy comes up to me and says, you wanna know how to throw a sinker, I was like sure. He said you throw it like a fastball, but you put frontspin on it. Personally, I don't think its right at all, and I tried it, just for kicks, and I couldn't get any speed or control on it at all. I would like to know how to throw a real one though, someone needs to tell us how.

im a sophomore in high school and im a lefty pitcher and i throw a total of about 7 pitches:
- 4 seam fastball (drop down and about 3/4)
- 2 seam fastball (drop down and about 3/4)
- change up (drop down and about 3/4)
- 12/6 curve
- sidearm slider (breaks from about 9-3)
- sinker
- knuckleball

u guys were looking for a sinker? there are a lot of ways to throw it:
i usually throw mine by holding in like a 2-seam and curving my index and middle finger down so it stays along the seam (not on it) and moving my thumb up on the ball and throwing it so that it has sidespin - i can get it to drop up to about 1-2ft down and if i vary it up a bit down and to the right

Try this link if want scientific help: http://hardballtimes.com/main/article/in-search-of-the-sinker/

Try this one for the simple answer:
http://www.baseballdrills.info/baseball/34a_cutfastball_and_a_sinker.php

For me, I grip it like a 2-seem fastball and move my thumb to the left (for a righty) so that it is not directly under the ball. How much you move it is something that needs to be figured out with experimentation. I use to throw across my body move (picture attempt: my arm moved more right to left than up to down), however I have recently figured out that I have just about the same movement with more velocity if I throw over the top. If you can throw a good 2-seem, you don't need a sinker. However, coming from someone like me who also has about 6 pitches, I would normally say that any pitch perfected, is a pitch needed.