How can transverse waves be polarised?
Have a look:
Basically you need a conducting grid that can "trap" one component of the electric field of the incoming unpolarized wave. For example:
The free electron in the metal rod of the grid "uses" the incoming wave
The case I showed you can be tested using microeaves and a piece of chicken wire (the wavelength of microwaves is comparable with the gap spacing of the mesh).
If you have, say, visible light of smaller wavelength you may use a grid of special polymers called polaroid.