I usually just cut the jug in half, poke a few holes in the bottom for drainage, fill it with a light, well-draining soil mix, sprinkle the seeds on top, and loosely replace the lid. The method that gave me the highest germination rate was making sure the seeds were pressed lightly into the soil but not buried too deep. Letting the jug sit outside all winter so it gets natural rain or snow seems to mimic the seeds' natural cycle perfectly.
One thing that can really ruin your winter sowing experience is forgetting what is in each jug. I thought I could remember which seeds were which, and then spring came and I had no idea. Now I write the plant name and the sowing date on the jug with a waterproof marker and add a little popsicle stick inside with the same info. I cannot tell you how many times this simple trick saved me from guessing what was growing.
As for soil, I use a mix that is half seed-starting mix and half coarse compost or sand. It drains well so tiny roots don't rot but still holds a little moisture so the seeds don't dry out in the cold. The soil stays fluffy enough for roots to spread and dense enough to protect them from harsh freezes.
I am in USDA Zone 6, and what I really like about winter sowing here is that it takes advantage of our natural freeze and thaw cycle. Even a short warm spell in January or February can help seeds start germinating without me babysitting them indoors. When spring comes and I lift the lid to see dozens of tiny green seedlings poking up through the soil, it feels like magic every time. Using a careful soil mix and labeling everything has made this method reliable and surprisingly easy. I would say start simple, write everything down, and trust the process. It really works and is so satisfying to watch.