I have mine done by the company. Although it is a bit of a faff. You have to go onto the company intrnet site, apply for a pack, wait for it come, fill in forms, make an appointment at one of the approved opticians.
I have a pair of specs specifically for PC use. They had to make them 3 times. The first time they assumed i had my screen at a certain distance. I couldn't see a thing. I measured the distance from my face to the screen so that they could make another pair and still got it wrong. Third time lucky!