Dear Claude,
The short answer is that you really shouldn't. In fact, in many areas, transplanting ferns from the wild is illegal, and with good reason: Collectors in parts of the globe have succeeded in driving several species to the point of extinction.
That having been said, if collecting nonendangered species is legal in your area, and if you have permission from the landowner, and if you are able to exactly duplicate in your garden the ferns' growing conditions in the wild (three big "ifs"), you may have some luck if you dig well away from the fern's crown in the early spring before growth is very advanced, transfer a large ball of soil around the fern and replant the clump without disturbing the roots in the slightest.
Still, this is a hit-or-miss operation at best, and I would recommend saving the time, potential trouble and considerable effort by simply buying your ferns from a local nursery.
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