All the windows I installed were single-hung Milgard Classic replacement windows which I think are now replaced by the Tuscany. The Classic windows had hollow jambs. On the top inside of the jambs were window stops that could be slide down. This is where I predrilled the holes to hide them. I used SS 1-5/8" Deck screws but the length would depend on the window thickness and what you are attaching to. To avoid breaking the vinyl I used my drill to get the screws in most of the way and then hand tightened them.
I did not countersink the screws since they were hidden under the window stops. I did consider drilling a larger hole in the first layer of the jamb so that the screw head would pass through it and hold on the second layer. This way I could use a vinyl plug to make it look better but I decided that it wouldn't matter since they would be hidden under the window stops.
I only used two screws to fasten my windows at the top-sides since the bottom of the windows had a tight fit between the outside stops and the stool.
Here are a few web sites that were somewhat helpful: