I start it inside every year to get an earlier start. I also do it because it is so easy to space exactly where you want, and it is fun to jumpstart your garden by putting lettuce/spinach plants in, instead of just seeds.
Here is what I do:
In Janaury, I usually start some Spinach, Swiss Chard, and Kale. Grow exactly like tomatoes, but no heat lamps/heat pads or any heating sources, and try to keep always moist. After they get tiny true leaves, I transplant them to slightly bigger coir pots. I disturb their roots without meaning too, and accidently snapped a spinach's stem, and they are still growing now!! Just try to be careful, but they usually do fine for me when I transplant them. After a couple weeks growing in their bigger containers, I start hardening them off. I leave them out overnight a couple nights before I plant them outside. And actually, today I am planting the spinach, swiss chard, and kale in my hoophouse and garden!
As for lettuce, it's basically the same exact thing but I sow them a couple weeks later, because they are not as hardy. I will probably start hardening them off soon.
Good luck! It's fun.
Taryn