Here's the structure of an "arbitrary" skill challenge I am going to include in my game.
Create a Magic Item.
0. Acquire sufficient resources. This means an amount of ritual components (the specific type related to the item to be created) that have a value equal to a random monetary treasure parcel of the item's desired level, and the base item - a sword, suit of armour, deck of playing cards, etc.
1. Imbue the components with magic. You take the components and mix them together while releasing magical energies into them. This step determines the abilities the item will have; this depends on your available spells, your level, and the desired level of the item. Acid Arrow could be used to create a sword that spits or coats itself in acid, for example.
Failure at this step can result in unintended abilities or simply the expenditure of the components. If the latter happens, the process must be repeated.
This step takes two days.
2. Imbue the item with the now-magical components. You bind the components to the item, infusing the item with magical energy.
Failure at this step can result in explosions, malfunctions, unintended abilities, etc. There will be a table for this. If the components or item is lost or destroyed, the process must begin again from the start.
This step also takes two days.
3. Seal the magic in the item. This makes the item permanent; failure might mean that the magic is lost after a single use, or perhaps the magic seeps out into other items and areas, causing all sorts of hassles. When a one-shot magic item is desired, this step can be skipped.
This step takes two days.
4. Engage in a battle of wills with the item. You either keep the item from forming goals and desires of its own or you shape those goals and desires to your own wishes. Failure means that the item is cursed; there's a table for that. If you don't care if the item is cursed or not, this step can be skipped.
This step takes one day.
Of course, if you have access to a wish or other factors that aid in creation of magic items, you may be able to bypass one or more steps.