Question

how does one get linearly polarised light with the help of a polaroid

Answer 1

HEY HERE IS YOUR ANSWER

when an unpolarized light falls on a Polaroid only those electric vectors that are oscillating along a direction perpendicular to the aligned molecules will pass through. thus incident light gets linearly polarised.
electric vectors which are along the direction of aligned molecules gets absorbed.
whenever unpolarized light is incident on the media the reflected light gets partially or completely polarised. when reflect light is perpendicular to the refracted light the reflected light is a completely polarised light.