You may have noticed how quickly the seed-starting soil dries out. Those seedlings are thirsty! But be careful not to over water them or keep the soil saturated, as this can cause seedlings to dampen off, which is when their stems become too weak and they topple over and shrivel up. I recommend watering seedlings from the bottom. Fill the tray so that the grow pots are sitting in the water, which is then absorbed through the bottom instead of trickling down from the top. This method encourages roots to grow downward in search of the water, which leads to a stronger, healthier plant. It also prevents the seeds from being disturbed by a sudden gush of water.
This may feel a bit ruthless, but thinning out your seedlings is an important part of successful growing. Wherever you see two or more seedlings growing right next to each other, pick out all but one and either discard the extra or replant them in individual containers. This will remove competition for resources and give your selected seedlings the space and nutrients they need.
If you started your seeds in one big seed tray, or in especially small plugs (tiny containers), you'll eventually need to transplant them individually into a bigger grow pot or larger plugs to allow for further root development and to prevent competition.
Planting out in the garden
There are two factors to check when it comes to planting out your seedlings in the garden. The first is weather; you need to be sure all chance of freezing temperatures has passed. The second is root development on your seedlings. Trying to remove your seedlings from their plugs before they have well-established roots will result in a crumbly mess and likely a destroyed seedling. You'll know your seedlings are ready when you see the roots have made it out of the bottom of the plug and have also formed a sort of web around the outside (as pictured above and below).
To remove your seedlings from their plugs, carefully tip them over a bit while squeezing the base of the plug. Do not pull your seedling out by the stem! Seedlings are fragile and yanking can easily break them. If their roots are well formed, seedlings will slide out of their plugs and hold a tidy shape (shown above).
Different plants require different spacing, so be sure to check the guidelines on your seed packets. Give your plants a very thorough soak as soon as you've planted them to help them settle in.