yeah..you know those long ones?

I know of three ways
1. Purchase Woodland scenic grass. Various colors, lengths etc.
2. Use the bristles of an old paint brush.
3. Take some hemp rope (twine) and unravel it and cut it to length.

All three are 'planted' in a bit of white glue and water mix (harder for longer grass) OR you can drill small holes in your base and 'plant' them in the holes.

I'm quite sure that if you browse the forums you'll find plenty info on this.

Anyway
I've read that a good way to represent these is cut the hairs of old brushes you don't use anymore. You know these brushes to paint walls etc - they already have the perfect colour. A bit drybrushing to vary the colours will make it just perfect.

u plant them one by one?

Typcially you plant them in small clumps. If they are short enough to stand up in white glue mix you can plant in clumps and spread them a bit with tweezers as the glue 'sets up'. If they are too long to stand up in the glue (gravity is the law ) and you have to use the drill hole technique. You drill A LOT of little holes close together and plant in smallish clumps. You need to consider what 'look' you're after. If you want a smooth 'full' look, use many more smaller clumps. If you want a more open look go with fewer bigger clumps.
You can also use shorter grassy 'transitions' to fill in the edges of the clumps to hide the open areas.
edit I will go back and add individual blades of grass if I feel the need to add that type of look to fill in areas.

I find it that if I drill holes and stick clumps of grass into them, the finished result is not very convincing. I prefer to clump the grass, dip the bottom of the clump in some while glue and stick on the base. That way I can spread the grass blades a bit to give them more realistic look. If I deal with tall grass I usually support the first few clumps with something so they don't fall over. Once the first few clumps are in place and the glue has set I then 'plant' the rest of the grass leaning it on the previously glued grass.

As it has been stated, using hemp rope, old brush bristles or Woodland Scenic's 'tall grass' will give you what you are looking for.

This is my entry for Overlord, you can see the tall woodland Scenic grass in use, I cut it down after I glued the grass into the holes I had already drilled.