Simple, because all you do is fill a jar with chive flowers, cover with a clear vinegar (I used distilled, but cider or wine vinegar would work, and possibly taste less sharp), and leave for a week or so.
By then the vinegar will have absorbed the oniony flavour of the chives, but more importantly it will have taken on their colour. All that needs to be done, is strain and bottle it. So simple. Doesn't it look wonderful? I'm still struggling to come up with a use for it - I could add it to salad dressing but the impact of the colour would be lost. I feel it's best served in a decorative vinegar bottle (my mother had several 'cut' glass ones as part of fancy cruet sets) but I've yet to find the occasion.