The milk cows are on a patrol circuit around the point on the nav map. They are not far from the point, do a hydrophone check and you should find them.
Once you have found a milk cow, get close and press escape, choose 'dock at U-XXX' and carry on as if you were at your home port.
FYI, if you have been awarded any medals or promotions, use them while at the milk cow as they will not be there when you return to base.
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