how to put 3d grass on a landscape

how do i put some 3d grass on a landscape in unity

Probably the best option to put grass on a landscape is to use the terrain engine, where you can add a detail 'grass texture' (there will be a grass texture in the project already if you have the standard assets), and then you can just paint grass all over the terrain.

There are some terrain video tutorials here. One of them includes grass.