Hi, the way I get those angled stitches is instead of drawing my thread doownwards, I draw my thread to the left and hold it there while I pass the other needle through.
This sets up a stitch that follows the angled line of the pricking Fork.
I aways stitch starting from the back, then as I said, pull the thread to the left, then stitch down through the front and pull both threads to tighten, then obviously start the process again.
You have to be consistent when pulling the thread to the side as swapping sides will make your stitching look uneven.
Also I've found it helpful to burnish or even file away the edges of the tangs on the pricking fork, you get a tool that releases from the leather a bit more easily and you also get a more refined hole for your awl to pass through.
Hope this helps,
SDB