Milk cows and supply ships are only active during certain dates. Press F1 and click "resupply info" to see when you can use a particular one.
If it is active, you can find it on your map as dots with blue labels.
You use one as if you were docking at your home port. Get within 25 miles (or go right next to it, if you are a realism buff) and press esc, then exit patrol, and then "Dock at: so-and-so"
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The entire German garrison of Vanviken, right here in your thread! 
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