Creating a Persona is a process quite separate from more focused areas of interest. Someone who is particularly interested in medieval cookery, or dance, or heavy combat, will be very likely to have a broad, cross-cultural and -chronological interest in that particular activity. In terms of more general knowledge, however, there is no way that one individual, barring unlimited free time, can achieve expertise in every time and culture in Europe in that period. Developing a persona therefore sets you a manageable goal, sacrificing breadth for detail in a smaller sphere.
Typically persona creation begins with selecting a time period or culture that interests you. From there, doing some research on the time period can help you flesh out your persona. Look at the activities, the names, the historical figures, the events, etc. that took place during your time period. This will give you a sense of happenings in the world during the lifetime of your persona (and it may spark a new area of interest as well).
If you so choose, you can begin to add particulars to the persona story. Your character’s history can be fairly simple, consisting of your SCA name, the time and place your persona is from, and a few facts such as occupation, social status and general family information.
Your persona’s life story can also be more complex and include a variety of factors such as: area/time of birth; languages spoken; occupation & craft skills; behavior/manners; pursuits/hobbies; weapons skills; travels; significant events during life (both historical and personal) and area/time of death. To accomplish creating a more multifaceted persona will take time and research. But do not feel pressured to develop a complicated story right away. You can start slow and add more details to the chronicle as you go along.
Additionally, there are questions that you can use in your research to assist you in developing a very detailed history. Examples of a few of these questions are: