This tutorial uses Photoshop CS2, but the techniques involved may be useful in other graphics design packages.
Start off by taking a screenshot, or use the following for practice:
I chose this ( http://www.airliners.net/open.file/1032067/L/
) for the grass.
Since this screenshot is taken in EGPH, I want grass from the airport where the shot is taken. So, lets start.
Open up both images in PS, and start with the real one. Select the Rectangular marquee tool which is marked in red (or press M) and drag it from the upper left corner (of the grass) to the bottom right corner (of the grass) and the whole grass part will be selected, like this.
Now, press CTRL + C to copy the grass, and paste it into the screenshot by pressing CTRL + V. It will look something like this:
This is not how we want it to look, so select the move tool (right beside the rectangular marquee tool, or press V) and select Show Transform Controls at the top of PS.
Now you see that you can "transform" the image to a smaller size, or bigger if you want to, but we want it small, so hold down shift and drag it from the upper left corner and down. (don't forget to have layer 2 selected).
It should look like this:
When you're done, press enter. This is still not the look we want so lets go to Edit -> Transform -> Disort.
Hold shift and start with the upper left corner and drag it down to where the grass begins, like this:
Do the same with all the corners and press enter.
This is how it should look like when it's (almost) done.
You don't need to do this next step, so if you're happy, just leave it, but if you still want to go ahead and add some "extra" grass, then continue with the next step.
Select the eyedropper tool (or press I) and select some colors on the grass, I chose #564923 as foreground and #6f5a3a as background.
Next step is to select the brush tool, and then select the grass brush.
Now, create a new layer, and lower the size to about 5 and hold down shift and just press once on the same spot where my cursor is, and release shift.
Then head over to the other side of the image, press and hold shift, and click where you want the grass to end. Instead of pressing and holding the left mouse button, click and drag it to the other side to make a (not so straight) line. Do that a couple of times, and when you're done, you'll have some nice handmade grass - plus some extra details (and not just a straight line).
This is how my final image looks:
You can add grass on the other side too, the steps are the same.
Keep in mind that these steps are just the basic ones. There's a lot more you can do, and there are other ways you can do it too, but this is the way I do it.
If there's something that doesn't make any sense, then feel free to ask!