First up, print out the guide available here or in the box to the right - ensure you print to size, not scaled to fit. For reference, the diopter boxes measure 2.9cm.

Take any glasses off that you may currently use.

Hold the printed guide 35cm from your face and start reading down the chart.

The last row you can clearly read indicates what lens strength you need - the corresponding number in the box is the diopter, or the optimal optical power you should select when purchasing your reading glasses.