Ive used a blue felt for the base so you can see the difference just for tutorial purposes.
All you need is.
Needle felting needles - ive used a multi needle felt tool. Its basically 4 needles in one tool and makes needle felting quicker. These are available on etsy, ebay and felting websites. They are also available to hold more needles
Here's a link so you know what i mean: http://www.etsy.com/search/supplies?shopname=Nikkinikkinikki72&q=needle+felting+tool&page=1 Look around though because you can pick them up really cheap.
Sponge. I bought this from a felting supplier but you could use upholstery foam or even a car washing sponge.
You can also get needle felting brushes, kind of like a large square brush. These are again available at the sites previously mentioned. But using the upholstery or car washing sponge will keep the cost down.
Here's a link to the brush. http://www.etsy.com/search/supplies?q=needle+felting+brush
Wool Merino tops. Here's a link to what i mean both on etsy. http://www.etsy.com/search/supplies?q=merino+tops Look for the cheapest you can get. There are sellers on ebay and etsy that sell packs with various colour. Look for single dyed tops if you want larger amounts of each. It doesn't have to be merino wool tops. Other breeds of sheep provide coarser fibres if preferred.
Its difficult for me to determine how much you would need. I would say 50g - 100g in weight would be enough to cover a 24 x24 inches piece of felt... probably much much more.
Piece of green felt. Thin as you can get it. You can buy the felt in most fabric shops and online
Placing the tops on the back of the felt and stabbing with the needles to make the fibres go through to the other side. Using three shades of green, but little flecks of a dark blue here and there would also be great!
Ive used the blue felt just for tutorial purposes, use a green felt piece when doing so or even brown.
Close ups to show you what can be achieved.
To make the grass use a piece of felt that's bigger than your planned area to cover. Kind of cut to the shape needed, but just a little bigger. Doesn't have to be perfect at this point.
Place the felt over the sponge and pull apart wool tops and place over. Try and use two or three shades for a subtle difference. Use mostly a dark green, with little hints of two other lighter greens here and there.
Start to stab the wool with your needle felting tool all over to hold in place. Doing so will make the fibres come through to the other side. Once all held in place with a few stabs here and there you then have to keep the tool going and stabbing down. I must have stabbed down about 100 times for small little patch showing in my photos.
Work from the middle of your felt outwards all the way around. The felt should shrink a little which is why i suggested cut the felt a little bigger to start with. Once you feel you have covered the area to the size required cut the felt to shape. Then place upside down again and just needle felt the edge all the way round.
You can then glue this in place.
I think this would be great if mixed in with soil and stones or even in tiny small amounts to represent moss.
The final piece is the reverse side effects of needle felting. Felters would use the side they have been stabbing into. Ive always been fascinated with the wrong side of needle felting and really do think its absolutely wonderful miniature grass.