There are exactly two gardeners who grow mandragora, you and me.
That's it.

If I were in your shoes, I'd simply plant the seeds in a pot
filled with reasonably open potting mix. Use a pot that is deeper
than normal for seeds to give growing room for the long taproot.
Cover the seeds with about 1/4" of soil or fine gravel; water
well; and park the thing in a coldframe. (The coldframe serves
more to keep excessive winter wet at bay than anything else.)

Expect some germination next spring, and with delayed germination
of sluggish seeds in later seasons.

Miller in the first edition of his Gardener's Dictionary (1731)
says that the seed should be fresh. You have been warned.

I would plant out the seedlings at the beginning of their second
season of growth, a year after germination. Surround the crown of
the root with fine gravel to keep water away, but even then, be
prepared for the plants to take several years to establish.

The crowns are prone to rotting off and it seems like they have
to rot, callus over, and re-grow several times before they settle
down and faithfully grow well. But once established, they are
steady performers -- not things of beauty, but something well
worth having if one likes unusual and curious plants.

My current mandrake planting is under the eaves of a large
deodar, which gives them some protection from overhead moisture,
but they much prefer a sunny site.


--
Rodger Whitlock
Victoria, British Columbia, Canada