As you can see from the picture, lettuce seeds are small!!! It's worth it though to plant only the amount you need, instead of dumping the whole package and then wasting time, money and seeds, by thinning them out. Well, maybe if you are a farmer dumping the whole thing is cost effective but if your like me and only need 4, it's better to just pick out four. Plus, I have a really hard time pulling out perfectly good little plants.
You plant lettuce, 4 to a square. Think of how the number four is shown on a domino. That's what you want. They only need to be in about 1/4 of an inch. I usually just poke a small dent in the ground and drop the seed in and cover. Two tricks for lettuce:
1) Plant only 2 seeds now, and then plant the other two in 10 days. It will stagger them so that you don't have 4 heads of lettuce ready at the same time.
2) You don't have to wait to pick the lettuce until it's full grown. Go out and cut yourself a salad. This way it's always fresh and has the most nutrients! As soon as it looks like it can loose a few leaves, cut as many outer leaves as needed, small inner leaves will continue to grow. You can keep doing this until it gets hot and the leaves start tasting bitter. Which is why you want to plant them now!!! You can also do another crop in late summer.