Question

How can transverse waves be polarised?

Answer 1

Have a look:

Explanation:

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 `vecE` field to move up and down practically cancelling it (using its energy) as in an antenna. So, the emerging wave will lose all the components of the `vecE` field parallel to the rod.

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.