Jaekaido, there's no definitive answer to this question, because there are many many variables at play.
First, think about the size of your districts. A district is basically an area that has a common feature (eg. the place where all the bankers live/work, or the area mostly inhabited by Italian immigrants). In medieval London, some districts consisted of just one main street, and then some smaller ones off it (see for example this map of the Jewish ghetto in Venice).
Second, the types of districts will be generated naturally by the history of the city - an area which, for a variety of reasons, has more poor people is likely to remain that way into the future. Similarly, an area that is associated with a craft - e.g. carpenters - will perhaps continue some tradition of that craft even if all the carpenters leave.
I don't think you should focus your districts so much on just one thing (eg. shopping, guilds etc.) because that's not how historic cities worked. It doesn't make sense for all the different guilds to be clustered in one place; it is much more sensible for, say, blacksmiths to live close to their guild, weavers to live close to their guild, glassworkers near theirs and so forth. Additionally, the location of a lot of these crafts will depend on the nature of the craft: butchers and tanners tended to live on the outskirts of cities, because of the smell and noise of their work. You'd probably also expect to see shops all over the city, rather than just in one district. While it is fair to say that some places might have a concentration of shops/markets, it's also unlikely that there'll be only one - in medieval London, there were at least 4 different markets (sometimes with a bit of specialisation). For example, in southeastern London, the fishmongers had their guildhall, and their market, all in the same district.
My recommendation is to think about who lives in your city, what are the main industries and groups of people, and why would some people prefer to live in one place rather than another (obvious example, very rich people probably don't want their houses in the middle of a poor district; immigrants from one country would probably prefer living close to each other, for protection and work)
PS: whenever I see questions about urban history and fantasy mapping, I always get a bit carried away with advice...
Wingshaw