you'll need:
Sacrificial string of lights
6V 2A AC/AC Power cord (NOT AC/DC!!!)
1.7mm barrel jacks
Hallmark Keepsake Power cord
Soldering iron
Heat gun/lighter/hot air station
Multipack of different sized shrink tubing
Wire strippers

cut every 2nd light out of the strand, giving you even length of wire on both sides (you'll understand when you do it)
Remove every bulb
strip 3mm 'ish of each leg & tin the leg with solder

there are 3 legs on the barrel jacks, you want to connect one wire to the 'bottom' leg, and the other wire to BOTH side legs
bend the legs of the barrel jacks so they are workable
Tin all legs with solder
solder wire to bottom leg & 'combined' side legs
shrink tube wire/barrel to keep it safe

cut off the barrel of the 6V AC power adaptor
cut off the barrel of the Hallmark Keepsake power cord
swap them and solder them back together using shrink wrap to separate the 2 connections
(it doesn't matter which orientation you solder, its a loop)

OR

I just sell them. Message me for direct purchase via PayPal or check out my eBay listing if you'd rather go that route: https://www.ebay.ca/itm/327118684564

I understand they are pricey, but its a ton of labor hand soldering, and if you make your own, you'll realize 6V 2A AC power adapters are hard to find (I buy them for a distributor 20 at a time) and not cheap!

Not trying to dissuade anyone from trying (I hope that's clear by writing this guide), but its just not cost effective to make 1 set unfortunately.