Measure both feet. Some people have one foot slightly larger than the other. Choose the size that fits the larger of the two.
Consider the socks or tights you’ll wear most often with your shoes. For thicker socks or tights, you may want to order shoes 1/2 size larger than you normally wear.
Measure your feet in the afternoon, when feet tend to be their largest.
If you require a wide width and no wide width sizes are available, try ordering a ½ size larger in the regular width.