Dig a hole in your potting soil deep enough, so that your lettuce seedling sits at the same level it did in it's nursery pot, cell or flat. You don't want to plant too deeply. Continue to fill the bag with the rest of your plants.
I tend to plant lettuce seedlings pretty close together because I like a container that looks full and bountiful. I harvest the lettuce by picking off the outside leaves, which keeps the container garden from getting too crowded.