First of all, let's look at what you can use to create the grasses. The thin rigger brush in the photo below is ideal for creating long, thin, individual wispy grasses, whilst various sizes of bristle brushes are perfect for producing short clumps of grass.
If you're not certain about this, just try the same technique with a pencil to give you confidence.
Various colours have been tried - don't just paint grass as one 'standard' green colour!
The grass on the left has been painted with paint the consistency of single cream, the ones on the right with thicker paint - hence the hit and miss effect and wider stalks....
The two clumps below were painted with a 1/2" flat bristle brush. The brush is loaded with paint then placed vertically on the paper.
Incidentally, the really ragged old round brush in the photo is great for this technique as well.
Now let's look at how to paint grass by scraping out with the credit card or the cocktail stick. This is more effective where you have a slightly thicker layer of paint.
Or another way is to scrape out the grass stalks, then put a light glaze over the area with your choice of colour....
And now a couple of thoughts about how to paint grass when it's a reflection. Note that the reflections of the light grasses are slightly darker than the grasses themselves.
Darker grasses would appear as slightly lighter. This is because the tonal contrast is always a bit less distinct in water. The grasses left and right have been produced with the rigger - the ones in the centre with a small bristle brush.
Finally, remember that grasses are usually in the foreground so to be effective they need to stand out.
Notice in the little sketch below of a rough stone wall I've put light grasses against the dark stone and dark grasses over the light bushes. See how the grass stands out very clearly.
Well there we are. I hope this tutorial on how to paint grass in acrylics has proved useful. Of course you can use the principals in any medium - not just acrylics.
And I'm sure you can come up with different (or better!) tools for creating grassy effects, so don't be afraid to practice and experiment - and keep enjoying your painting!