Find the armor trim template you want to duplicate
Gather 7 diamonds
Gather 1 block of the template’s matching material
Place the template in a crafting table
Place 2 diamonds in the remaining slots on the top row
Place 1 diamond in the center slot
Place 3 diamonds in the remaining slots on the middle row
Place the matching material in the bottom center slot
Take the duplicated armor trim template from the result slot
Repeat as needed for more copies