Upon selecting the trading option, a new window will appear. On the left there will be a list of resources available, along with their prices and an icon indicating if you need any of them for your menu. The prices differ between locations, so you may want to try to find the best option available. You can click the list icon above the list to filter only those resources that you need.
When you choose an item from the list, you can see whether and how much of the given resource you already have. You can also see its average price, depending on prices in other locations.
Clicking on the icon below the list will add the given item to your Shopping list. You Shopping list has limited space, indicated by the number below it. Next to it you will see a total sum you will pay and the cost of transport. You can also choose the Express option, which will increase the cost, but the supplies will arrive sooner.
Before paying for the resources, you can also use the economic (green) gossip to get a discount.