You can place a seed in soil and place it directly under a light that is scheduled for 12/12
Keep in mind that if you are growing an Indica strain that you are going to end up with a very small plant. I have around 20 plants that went straight into 12/12 and they gave me just under an ounce of dried herb per plant. I am going to veg longer now because G13 Haze does not produce very much bud going 12/12 right away and I do not want to sprout 40 ladies and only harvest 1.5-2 lbs. So for personal use or if you have access to hundreds of seeds then 12/12 is great, but if you need to get the most out of the seed stock you have then veg for 2-4 weeks and then flower. I tried 12/12 and will do this for my own personal bud but I supplement my income with ganja so I am going to veg for 2-4 weeks or until I feel the plant has enough bud spots and then place in my tent. I don't pinch buds from my stash and only smoke the herb from the plants my girlfriend calls dibs on
If you veg for a while you will yield 2-3 times the amount of herb than if you went directly into 12/12.