Visit a library. Examine anime books from the reference area.
Find books of armor. Dorling Kindersley's "Eyewitness" series has excellent pictures of armor from all eras. Copy pages of interest. These are for your reference only -- the book's pictures are copyrighted.
Examine manga art similar to the style you desire. Watch anime. Take notes and make sketches of the armor on the characters.
Attend a reenactment of armored combat. Take a sketchbook and camera to capture the combatants in armor. Organizations hosting these events include Society for Creative Anachronism, Live Steel groups and role-playing groups.
Create the character who wears armor. Develop a background story for the character. Characteristics of the armor, personal body style and economic status of your fictional person drive the armor's appearance. For example, Spiderman's "Spidey-suit" was created from red long-handle underwear.
Draw the character in many positions. Determine how wearing armor affects the way a person sits, stands and walks.
Add armor to one of your drawings. Base the armor on your research, but experiment with embellishing and personalizing the equipment. Will the helmet have a plume or a spike? Will the armor be plain or ornamented? Draw each piece separately so you can try them on the character without having to redraw everything.
Complete drawings to use as prototypes of the figure wearing your armor design. These serve as templates for your anime production.