I don’t know all the items in the book and don’t have it in front of me, but I imagine a Staff of the Magus that allows storing and and all spells in it and gives bonus to magical effort.
Or a set of earrings or rings that take up one slot, are difficult to steal when worn, and store 10 spells.
EDIT: Sorry, I didn’t see you were asking for rules on how to create items. There are no hard and fast rules and in my games, having a “crafting system” is always tedious. What I do is use the Narrative to figure out if the players can create something and if they have the right materials for it.
My rule of thumb is letting them know that they found special material (Example: Blood Gems) and that they could probably start creating a new set of rings or earrings to create something (Example: Mage Earrings from above). Depending on how much power or properties the item they are asking for is, I will set 1, 2, or 3 (3 being a super powerful artifact they are trying to recreate) threshold requirement.
When there is downtime, it is narratively appropriate, and when the PC can dedicate enough time on it, they can spend Coin or other resources of your choice (based on your setting) to roll EFFORT (no attempt, as they are in downtime and usually safe). This can only be done once per downtime session (not in-game time, but that is just how much they can progress in that span of time they spent in Downtime). Each time they roll effort, they have to spend some Coin or resource (this helps determine the price range [more skillful artisans are more efficient] and erratic behavior of creating magical items). If the players want to spend more and more resources in one go, they can probably get a bonus.
Just something to think about.