A common mistake in the Society is assuming that modern English pronunciation and spelling rules can be applied to medieval names. Surnames were first used in Western Europe starting in the 1300's and were generally used there by about 1500. There are basically four types of bynames. Most cultures have examples of them all, but each culture has certain patterns which stand out as characteristic. Locatives, based on a place of origin.
Epithets are terms which describe a characteristic of the person.
The first three groups are pretty easy to document. If you find a period example of a name, place, or occupation, you can use it as the basis for a patronymic, locative, or occupational name. You may have to make some grammatical changes but you can be fairly sure that you've got the right idea. Epithets are trickier because not every descriptive phrase is likely to be a period descriptive phrase. Medieval people weren't any different.