The rules are optional. A character needs a formula and possibly special materials and locations (left to the DM). Common items cost 100 GP and require 3rd level; uncommon 500 GP and 3rd level; rare 5,000 GP and 6th level; very rare 50,0000 GP and 11th level; legendary 500,000 GP and 17th level.
A day of crafting (8 hours of work) is worth 25 GP of progress. If the item will cast a spell, the spell must be expended every day during the crafting, but of the spell can be cast once (such as a scroll) the spell needs to be expended only one time. Multiple characters that meet the level prerequisite can cooperate, each contributing 25 GP per day and, if needed, spell slots.