What I do not understand though, in that article, the seedlings have a brown stem, but the pines length are short (unlike mine with green stem and long pines).
Plants with too little light elongate. Pines need loads of light, which is why sowing them in winter is a bad idea. In summer light is stronger.
Timing the cutting is based on the color of the stem: As the stem matures, it will change color, going from green to purple-ish to brown. When they are this red/purple is the time for the cutting, before they really turn woody/brown.
I am not sure whether it was here or in another thread: If you are this new to growing stuff I would NOT try this technique. It adds a little to the final product, but your concern should be getting this to grow out. Next year do another batch to play with more advanced techniques I would say.
As it is a delicate task, most people don't want to tell you to do that, because it depends much on your experience if that goes wrong or not, so none want to kill your JBP seedlings by advice... xD
But separating seedlings is actually not that hard, seems like you did just fine. sometimes you have to risk a bit to learn, and when you have only a few trees/seedlings is hard to keep up with the idea of loosing it, the more trees you have, less you'll care about making a mistake, and so, more you'll learn...
Depends on the soil your using.
Hard to figure out from the picture what your soil is underneath that first layer.
Normally i water pines less then other species.
But also depens on how they are taking the repot.
The soil is 30% akadama, 30% pumice, 30% lava rocks mixed.
I've placed 2 empty rolls in the pot and filled it with the mix from above.
I've filled the rolls with vermiculite and planted the seedlings.
The layer on top is extra vermiculite. I've only placed for aesthetic reasons.