Here's what I coached for about 20 minutes with each pitcher doing so on his own.
-Put the ball in your glove, sitting high in the pocket, with your four seam fastball grip.
- Now here's the part that you have to practice, to the point where it's a matter of "feel confirmation" not vision.
( In other words don't look into the glove.)
-Now use your thumb to rotate the ball so the index and middle finger actually feel the grip that you've decided to go with.
-Again, don't look into your glove. When you look into your glove you're giving away you pitch selection, event though that selection is not visible.
Practice makes perfect here - over and over and over. So, take some time to be somewhere, anywhere - watching TV, sitting on a porch, your front steps, anywhere - and slap the ball in your glove... then reach in and feel for the four seams as if you're about the grip a four seam fastball.... then take your thumb and start to rotate the ball anyway you want to feel the two seam, slider, split finger, curveball, change-up/off-speed....
The entire process here is to work quickly and effectively. You get on the rubber, get your sign, reach in and "dial" your grip without hesitation, then send it down range.
You'll soon develop a greater skill, going into this routine than you realize, Soon you'll understand the causes-n-effect of grip Vs ball dynamics and how "feel and sensitivity" with playing the seams is what makes you a better pitcher.