People learn best by doing, so let's begin by doing stuff!
Place a chest and fill it with cobblestone. The shop will take items from this chest.
Write a sign like this:
YourName
64 cobble
$0
using your own name instead of YourName. Make the last line blank.
You should see a helpful message. If this doesn't happen, break the sign and try again.
Type /duckshop link to connect the chest. Follow the on-screen instructions (punch the sign, then the chest).
The shop should be working now. Right click the sign to get a free stack of cobblestone.
This just scratched the surface of what DuckShop can do. For more info, take a look at the rest of the documentation, especially the examples.
Happy trading!