Concept and Role
Source GM Core pg. 134
First, come up with a concept for the item based on the role the item serves in your game and in the game's world. You might include a new item in an ancient ruin to hint at its history and characterize the technology lost when its creators disappeared. For instance, a ruined starship of the First Ones might have an item related to the construction of anacites, while an abandoned arcology on Bretheda might have medicinal items developed using biohacking technology lost in the Gap.
A new magitech item might be important later in the story, or its role might be as simple as a fun shadow-themed item for the witchwarper that uses Twisted Dark Zone. Keep your concept in mind to guide you through the process. Start thinking about what kind of magic item it will be. Each item type has its own niche, and some are less likely to be as useful to the PCs. For instance, new weapons and armor require the PC to give up the weapon or armor they already have, which might make them more reluctant to use the new items unless they're noticeably better, while consumable items don't have as big an impact on the story as permanent items.
A new magitech item might be important later in the story, or its role might be as simple as a fun shadow-themed item for the witchwarper that uses Twisted Dark Zone. Keep your concept in mind to guide you through the process. Start thinking about what kind of magic item it will be. Each item type has its own niche, and some are less likely to be as useful to the PCs. For instance, new weapons and armor require the PC to give up the weapon or armor they already have, which might make them more reluctant to use the new items unless they're noticeably better, while consumable items don't have as big an impact on the story as permanent items.
Item Level
Source GM Core pg. 134
A new item is typically going to be within a few levels of the PCs. If it's too low, it might not be interesting, and if it's too high, it might be too powerful or too lucrative to sell.
Comparison
Source GM Core pg. 134
First, look at similar items. For example, if you want a permanent item that lets someone fly, look at jump jets, which only boosts a character into the air for 1 action and thus can't be used to gain a huge advantage in combat, and jetpacks, which can. This will give you an idea of the right level range and the specifics and limitations of existing items. You might be able to just adjust one of those to get what you want.