First thing you need to do is locate the seedling you want to move. Then, as shown, slide your knife down and under the seedling, then lift the seedling out. If the weather is cool and the soil is cool and moist, the seedling won't be too upset. You don't even need to take the entire root clump, if it can be called clump at all.
This was a particularly skillful removal. With this kind of weather, that seedling would be fine just sitting there, on top of the soil, for several days. Instead, though, I found an empty spot and moved it. The spot was empty because scatter sowing is not a very exacting way to sow, not very exact but also not a problem once you learn how to move seedlings.
And there she is in her new home. To finish the move, you more or less repeat what you did to extract the seedling: slide your knife into the soil, push the knife to one side or another, creating a hole, plop the seedling into the hole, then tamp it down a wee bit. There is something really tender and amazing about moving seedlings. Moving them around, handling their tiny stems, roots and leaves, requires delicacy and care, and it feels good to exercise those human qualities. Oh, and if you don't have a pocket knife, a regular old butter knife will do.