A note on seeds: Even most small seed packets come filled with far more seeds than one average gardener can use. Save more money by splitting seeds with a friend! You can also save seeds from many plants in the fall and then plant them the next spring. Visit WinterSown for more information and ideas.
Once you have your seeds planted, use duct tape to tape the jugs closed.
I generally put a plant marker in each jug, but I also write whatever the plants are on a piece of duct tape and attach it to the bottom of the jug, just in case it fades too much from the sun.
As long as there is condensation on the inside of the jugs (as shown in the top picture), you don't need to water the plants. If they look dry, either spritz or drip water in until all the soil is moist.
According to what I've read, if you live where it's colder, you can leave these containers out all winter in the snow and everything. We don't really have much winter, so ours grow pretty fast. Once your plants are big enough, you can remove the duct tape and let the little plants get some sun during the day, then just tilt the top back over for protection at night.
I don't follow any hard and fast rules about when to transplant from the containers into the garden soil, but generally if the plants have some secondary leaves and the weather is good enough, and they have been "hardened off" by having the tape removed for several days or a a week, I will transplant. The little swiss chard seedlings in the photo above will go into the ground sometime this week.
Another one of my favorite containers to plant in is foam cups with clear lids. These are great for plants that need a little more space, like tomatoes. You can plant just a couple seeds in each of these. There is a great step-by-step tutorial at WinterSown for these.