i am new to zen-cart. i would like to add new sidebox to let customer redeem the gift voucher after login. but i am not familiar the zen-cart coding structure at all

anyone can give some idea which files should be added and modified?

i try search on the documentation of zencart on website. but there is no documentation of the system design and coding. If you have found any can you please share the URL here

is it very diffult to customize zen-cart?