Back in the UK, in my native Surrey, we had a small Walnut tree that regularly had a good crop. As Walnuts had difficulty ripening in Surrey, and to prevent them being eaten by Squirrels, the crop was invariably picked early, and pickled.
It is said by epicureans the world over, that the finest accompaniment to a really good mature Cheddar Cheese is a pickled Walnut; and I must agree.
The making of these pickles is a very simple, but lengthy, affair; you will probably not be sampling them for up to two years; by which time they will be superb. The ones below are from 2018.
It's not complicated. They must be gathered before any wood begins to form inside. Now is ideal, as long as they are of a decent size. Traditionally in the South of England we would pick them during Goodwood Week; but I expect that's been cancelled this year.
Firstly they are trimmed of their stems, etc (topped and tailed), and any blemishes removed, each nut must then be pierced a couple of times with a stainless blade; at the same time ensuring that no wood has started to form.
They are then bathed in brine for two weeks (changing the salt/water every so often), drained, then put out in the sun for a couple of days until they turn jet black. Then they are packed into sterilised jars, and covered with either malt or red wine vinegar, a reasonable amount of sugar, and a few peppercorns. The Walnuts should be totally covered, and left for up to two years before eating (one year is OK).
None of the details is exact... just use your instinct (or consult a pukka recipe, such as the one below).