First, you have to choose the usage of your eyeglasses, for seeing near/far/intermediate distance. Then, you have to enter the prescription for your Sperical Power(the value are usually showed as a positive sign"+" refers to farsightedness and presbyopia), Cylinder power & Axis (this value is available only if you have astigmatism), ADD=Near Addition, (important value for reading glasses)means the power strength to be adjusted with SPH number for making single vision reading glasses, computer glasses, bifocal and progressive glasses. Some wearers may find their ADD value are put under the "N.V." on the prescription. Please enter the same ADD for OD and OS, if you have only one ADD value.
Learn more about how to read prescription at http://www.firmoo.com/help-p-27.shtml
It is the same as you buy regular glasses. Although the form of prescription are different from different online glasses shop, they are the same in nature. You just need input your prescription online strictly according to the prescription given by eye doctor. And also, you may need to choose usages or tell them that you want to buy reading glasses. Then, you can continue the order as you buy other items online.