I did some simple basing on my handgunners and for the time I spend on it, I think it turned out ok. Most of you probably know this method, but in case someone doesn't, I'll go through it really fast.
It is really very basic (haha), I cover the base in PVA glue, then I cover it with some GW modelling sand and flick the bottom of the base to get the excess sand off and then let it dry for a while.
I then make some smaller dots of PVA glue on the sand, I take a small ball of GW grass which I then place on top of the glue dots. To make sure it sticks I press it down slightly and if you like you can also try to get all the grass pointing in the same direction. After letting it dry for a bit, just flick the bottom again to get the last loose grass and sand off.
Ok, that wasn't at all fast, but it's faster to do it than it is to explain it.
This is the result. Not a great picture I know.
I sometimes mix the GW grass with some other types of grass, for instance the Army Painter steppe grass which is darker and more curly. This will make the grass look more downtrodden.
I know that painting the base, either before the sand is glued on or after, also is very popular, but I like the look of this easy method. And if you're a bit clumsy like me, you don't run the risk of getting paint on the already finished mini.
PVA Glue by the way is the same as normal wood glue.