Pour the spirits (vodka or brandy) over the walnuts in a canning jar.
Set aside until September
Gently pour the liquid off of the nuts taking care not to disturb the sediments
Add the sugar and mix it into the liquid
Let this sit for another couple of months
Blending the liqueur
Strain the spirits off of the nuts into a new clean jar, either by carefully decanting, or by pouring it through a coffee filter or clean white cloth
Add more sugar if desired, 1/4 cup at a time, allowing it to fully dissolve before adding more until you find your ideal sweetness
Set the blended liqueur away for at least another month, ideally for as long as a year. The longer it ages, the mellower it will be and the more the walnut notes will mature.
Notes
I've tried more complicated methods, but this simple one also produces the best results by far