Once your seeds have their first set of true leaves, it’s time to transfer them to pots if you’ve started them in those disks of compressed peat. If you’ve started them in little pots, you can wait until they have two sets of true leaves of more.
Wait a second–what’s all this business about “true” leaves? Well, you won’t know until your seedlings have two sets of leaves. Then take a look at the first set. They are most likely very round and different from the next set. That’s because they are not “true” leaves; they are what is known as cotyledons or “seed” leaves. These are the very first leaves the seedling pushes out to begin the process of obtaining chlorophyll and making food for itself. They are not in any way what the “real” leaves of the plant will look like.
Once your plant is ready to be potted up, it is most important again, that you use only seed starting soil (Gardener’s Supply actually sells an interim soil for this purpose but even I don’t go so far as to use it). Again, regular potting soil is going to have fertilizers in it and they will be too strong for these tiny seeds and will burn them and kill them.
Use only clean pots (you don’t want to risk contaminating the soil and transferring fungi that might cause damping off, now, do you?) And handle the tiny seedlings by the stem, which is sturdier than the little leaves. If you break the entire top of the plant–no more plant. By grasping the tiny stem–gently of course–you are much less likely to do damage.
Place the rooted stem into moistened cutting mix and keep the mix moistened–remember Friday’s “What Can Go Wrong?” post. These seedlings are still prone to drying out, and once they have dried out, they are dead. But evenly moist, if you can manage, is the best practice. A small watering can with a tiny rose attachment (the head that allows you to sprinkle water gently) will accomplish that.
Years ago I bought 4 small screw-on caps for soda bottles from a catalog company called Lee Valley (and no, I get nothing for mentioning their name.) I save them season to season and they allow me to have a couple of bottles available, with water in them, at all times, for “drought emergencies” with my seeds. I notice that now their collection includes one for “weed watering” (presumably with some sort of herbicide, be it chemical or organic. It’s a nice collection and it sells for $4.95 plus shipping.)
By carefully transplanting the seedlings and keeping them moist, you should be ready to set them outside when the warmer weather comes!