Here’s an article on making sashimono for your Japanese feudal armies. The technique actually can be applied to every period in need of flags (that is, virtually all of them)
Bushido New Dawn is a set of free wargames rules for Japanese Fantasy/Myth skirmishes.
Koku is a set of basic Samurai warfare rules with a mechanism for purchasing your troops.
The prolific Richard Bradley has a set of free wargames rules for samurai skirmishes.
No Dachi is a set of free wargames rules for Samurai skirmishes.