The key i've found to making grass tiles, is to start with a recognizable shape, mess with it, and then tile it, preferrable making a few random variations, but few chanegs to the original shape
hopefully this image of me screwing with 2 shade-grass tiles in 16x16 tiles (i noticed that was the size you were using)
(you'll notice how the 1 tile grass is almost more effective than the 4 tile grass, though those were quick rotates.
of course, that's the plain 16x16 tile i've got to the left, a few touchups and it could tile seamlessly.
If i may say so myself, the tile is rather flat, but sometimes you can't care about a grass tile, they need to be flat (low contrasted, the majority of my highlight color is a hue shift, very little shift in luminoisty) to function correctly.
You want to, keep the majority of blades (if you draw a tile in this style), the exact same shape. The eye which spots where a tile grid happens, preys on little dark spots, so you need to even disperse inconsistencies, or have none.
edit: overall, you're not going to be able to be taught, you can take the principles of many, analyze them, and try to learn yourself. It's rather hard to explain a grass tile alone.
Though i hear they're pretty big, it's the only tile i've got my head wrapped around at all