There's rarely just one correct way to grow bonsai.
I've used a number of different strategies with young pines and all work.
Cut around 5 cm tall to promote more really low branches. Some will become sacrifice branches to thicken the lower trunk. One will make the new trunk and one may be a low first branch. The bit you chop off can be rooted as a cutting.
Wire the trunk and add a bend or 2 while the trunk is really pliable. While wire over needles does not look attractive you are not intending to show it and I've never seen any adverse response to wiring over needles.
Let it grow as is for another year. You'll get best thickening from just letting it grow but sometimes not the best trunk to work with later. Pines are quite flexible for several years so it can still be wired and bent next year.