This is as easy as assigning a difficulty and making a roll.
If the Roll is twice the Difficulty, the item created is a superior product, and grants a +1 bonus to any who use it as intended. If the Roll if 3x the Difficulty, the item is +2 and so forth. Less than 5% of items are superior.
Here are some general guidelines:
Easy: Paper airplanes. Children's crafts. Coffee.
Medium: Anything from a kit with good instructions. Furniture assembly. Eggs. Basic painting. Installing doorknobs. Basic pottery. Making a website in Google Sites.
Hard: Assembly with poor or incomplete instructions. Installing doors or windows. Basic computer programming. Default for most hobbies. Default for most common cooking.
Very Hard: Default for most construction trades. Default for mechanical engineering and black-smithing. Database management programming. Default for farming. Default for preparing real cuisine. Default for most fine art.
Extremely Hard: Anything that requires actual calculus. Software system engineering. Default for electronic invention. Default for chemical invention.
Brutal: New materials invention. Default for AI programming. Default for robotics or cybernetics.
Unbelievable: Creating magic items or other artifacts. Default for any item that otherwise could only be purchased by Fate.
Impossible: Anything involving the very fabric of time and/or space. Anything involving gravity control.
There are a few factors to consider: