My ideas were bad, e-aksenov's had much clearer suggestions:
Part 1: Design
A long term project, usually 8 segments, based on STUDY. It is either to Invent a new design or to Learn an existing one
If Inventing a New Design:
‣ GM Asks: What type of creation is it (Mundane, Alchemical, Arcane, Spark-Craft) and what does it do? (Player answers.)
‣ Player Asks: What’s the minimum quality level for this item? (GM answers, using Magnitude as guideline.)
‣ GM Asks: What rare, strange, or adverse aspect of this formula or design has kept it in obscurity, out of common usage? (Player answers.)
‣ Player Asks: What drawbacks does this item have, if any? (GM answers.)
If Learning an Existing Design:
‣ Common Creations (p. 226): Do not require special formulas or designs to learn.
Part 2: Creating the Item requires 1 downtime activity + a Crafting roll
Dice pool:
‣ 1d for each Tinker action dot.
‣ +1 quality per coin spent.
‣ +1 quality for Workshop crew upgrade.
Result:
‣ 1-3: Quality level is Tier -1.
‣ 4/5: Quality level is equal to Tier.
‣ 6: Quality level is Tier +1.
‣ Critical: Quality level is Tier +2.
Modifying an Existing Item: Requires 1 downtime activity + a Crafting roll, no designing or learning needed
Minimum Tier = item's current Tier +
‣ +1: Simple, useful modification
‣ +2: Significant modification
‣ +3: Arcane, spark-craft, or alchemical modification
Drawbacks: GM may assign 1-2 Drawbacks to a creation