Are we talking about hammers or banners now? (they have both
)
The banner usually has some form of the orders insignia, or a reference to it's mythology. It may also contain signs of a certain chapter within the order.
There are smaller ones that are mounted on the back of the marine's armor similiar to the once japanese used in the middle ages). Or bigger ones to be carried around in hand, more alike european middle ages style.
You would want to pay attention that the colors and signs on your armor and banner match, otherwise face ridicule by certain WH40k nerds.
For picture references... there are millions of them on google search.