Simple. Put some water in the sink, add a small amount of dish soap, mix until it’s soapy then add the lettuce. Iceburg lettuce is all we had when I was a kid so washing the outside was all that was necessary. Nowadays, all types of leaf lettuce, spinach, greens, etc. are common. You wash them the same way. Mix the leaves around in the soapy water a little and then rinse clean. My wife does it all the time with her vegetable garden lettuce, spinach, swiss chard and collard greens. I have to wash individual celery stalks off from dirt that collects at the bottom of the main stem before I can eat it. It’s not that big of a deal. Of course, lettuce or cabbage cut up and processed in sealed bags nowadays has been pre-washed before packaging. But if it is unpackaged, raw produce, it’s wise to wash it before consuming it. Just think of how many people pick over cucumbers, apples, etc. before they pick what they decide on. Even the grocery worker has handled the produce when they put it out for display. Unsanitary is in the eye of the beholder. Some people never clean their produce and do just fine. It’s a matter of personal preference and choice. To each his own.
Of course, lettuce or cabbage cut up and processed in sealed bags nowadays has been pre-washed before packaging. But if it is unpackaged, raw produce, it’s wise to wash it before consuming it.
I still wash the bagged stuff. I don’t know if there’s a chemical or something added, but I detect a strange odor/taste on it.