I agree with Dee. One year, when I was planting crazy amounts of seed, I did splurge and purchase clear plastic ”to go” containers from Gordon foods. Those worked very well, after drainage and ventilation holes were added via soldering iron. But, milk jugs are far less likely to tip over, and more seeds will fit in each container.

Have you found the Winter Sowing FAQ page? If not it’s here https://www.gardenweb.com/discussions/winter-sowing-faq

If the idea of a soldering iron intimidates you, try a metal skewer heated on your stovetop!

Soldering iron is easy, fast, and efficient. (I've tried many options and this is my favorite.) Just use outdoors because of the order and you don't want to breathe the chemicals!

I’ve always used a hot skewer too. I don’t own a soldering iron.

I bet the soldering iron works great! I do not own one and I have to prepare my jugs indoors. I use a very lightweight drill and a rasor knife when preparing my milk jugs. I bought myself the lightest drill I could find for this task. I use milk jugs and jugs from iced tea that have a flat bottom but I have also used some nice used fruit/vegetable containers. I love when I get to open the containers and let the seedlings get sun and air in Spring. Wintersowing is a great thing!!!

I actually just use kitchen shears to cut four slits in the bottom of the gallon milk jug when I’m cutting the top.

Gallon milk jugs only, I dont use tape or leave the caps on. The translucent plastic works better than clear pkastic like juice bottles. I do love big salad mix containers for starting seedling on heat mats, but I digress.

Start with perennials and things that self seed in your area.

Use good potting mix (NOT garden soil) - I like pro mix.

Great way to get a ton of plants cheaply (like if you are doing a new perennial border). I dont think it gets you early starts (for things like peppers or petunias) but nothing better for huge numbers of cleome, columbine, coreopsis, coneflower, marigold, African daisy, rudbeckia plants for almost nothing. Pretty good for tomatoes like yellow pear, but we have a short growing season (zone 5 upstate NY) so I start most hot weather veg indoors.

I use the technique during the growing season, too (milk jug greenhouse in shade) for lettuce transplants and stuff that surface seeds and likes moisture (lobelia)

I started using a soldering iron but then just went for slits with a Swiss Army knife, much easier and I don't smell burning plastic.

One thing I learned--if you use those large grape containers or salad containers that have MANY ventilation holes around the top AND bottom--you'll want to close most of those up as I found those dry out pretty quickly while the jug-style containers stay nicely moist.