When your seedlings have sprouted in their germination tray there comes a time when you need to "prick" them out. This involves loosing the medium from around their stem and freeing their root system from the seed starter mix. This needs to be done very carefully. When handling the seedling pick it up by the leaf. It is better if the leaf breaks off than if you would apply too much pressure and crush the stem. I put enough potting mix in each cell of the pack to fill them up about half way. Make a hole in the mix with your finger or use a dibble. The dibble I use is a customized one that I made from the handle of a steak knife. Then drop the seedling roots into the hole and firm the soil around it. Finally spray with water to settle the seedling in. Here is a video of the process.
PRICK TO PACK
As you can see, that was pretty easy to pack the lavatera. I hope you noticed how I lifted the seedling by the leaf but also the advanced development of the root system. I probably could have packed that seedling sooner. Usually I can prick out enough to fill a pack. However, only one other lavatera has sprouted out of the 5 that I have planted. I will prick that one out and then start on the New Guinea Impatiens. Again, you don't want the seedling's root system too advanced because it will make it hard to prick it out and also to fit the roots into the individual cells of the pack. Some folks direct seed into the packs. I have never had very good luck with that method. When I tried that I had leggy plants. But it can a be a real challenge to prick out the smaller seedlings like impatiens. Some flowers just won't tolerate this at all. They do not like their developing root system disturbed. Last year I started some petunias inside. The petunia seeds are so tiny that they are pelleted for ease of planting. Those seedlings were very hard to prick to pack (P2P).