Your seedlings will have been growing under the lights and you will have been watering them each day, maintaining an even amount of moisture. But once your seedlings look like this, they need bigger containers. They need to spread out and develop their root systems. Because different plants develop at different rates, seedlings will be ready at different times. Transplant out the ones that are ready and let the others have more time. These are my 4 groups of Hollyhock seedlings. I am planning a bed of Hollyhocks, fingers crossed. You can see that some types are further along then others. This is normal and no cause for concern.
When transplanting, plants are to be given more space further apart from each other, and most plants are to be buried at their current depth, except tomatoes. When I move tomato seedlings to a bigger container, I bury them up to their first leaf set. The stem that is then buried will develop additional roots, thus allowing the plant to become a much stronger one. It may feel like you have made them smaller, but that is only temporary. They will grow very large, very soon.
Here are tomato seedlings, buried to their first leaf. Also, you want to give them plenty of space between them to grow. They will want to be couple inches apart.
Plant all other seedlings at their current depth, like these tiny Lupine seedlings. Then set the larger containers of transplants under another grow light, one which will be set higher on books so that the light is 1″ above the tallest leaves. With more space, they will take off pretty fast.
Here are the tomatoes and lupines only 2 weeks later.As these continue to grow, just add more books to the ends to raise the light. They will also drink more water each day. You don’t want them to dry out. Now that these seedlings have moved on to these larger containers, you can be starting more seed on the grow mat. Good luck!