Tap the recipe in the list. You'll see detailed information about the final item and the components required to craft it.
If there are enough components, they are marked with a green check, and the Create button is green. If any component is missing, you will see a plus sign next to it. Tap it to purchase a component or learn where to find it.
To start crafting an item, collect all components and tap the Create button. If it requires time, a bar with time until completion will appear above the recipe list.
Keep in mind: you cannot craft several items at once, so all categories have one timer. When nothing is being crafted, the bar indicating time is empty.