|Art by dleoblack - source.|
Piecemeal armor is built from diverse bits and pieces. Each part might have a different quality, weight, and material. Some are improvised and asymmetrical. This is suboptimal and only common in post-apocalyptic worlds where you cannot find a blacksmith to make coherent armor.
The rules in chapter II are abstract enough to cover these kinds of armor. The only exception is that you do not usually buy this type of armor, but build it over a long time, as you find useful pieces.
As an optional rule, every time you find armor with equal or higher cost than yours (that you cannot simply steal for some reason) or other useful parts, you can make a check with the relevant tools (leatherworker’s, smith’s, etc.) to see if you can upgrade your armor by one step (changing AC, weight and price accordingly). The difficulty is equal to the armor’s current AC.
This can take a while. A quick rule of thumb is adding the result of your check (in gp) each day to the value of armor until it reaches the value of the desired armor type. If you want to upgrade a weapon from a 30 gp value to 50 gp, for example, you need to roll 20 or more. Three failures usually mean you ruined the material. Armor made this way has little resale value.