Sounds easy so far, right? Good. While the spaghetti is cooking, cut the bacon and onion into little pieces. It doesn't really matter how big or how small the bacon pieces are, do you think the soldiers really cared about that back in the day? :P
Next, crack one egg and one egg yolk into a bowl. Depending on the size of the egg, you might need two eggs and one yolk. Beat the eggs and then slowly pour in the grated Parmesan cheese into the bowl. Continue to mix the cheese and eggs until it thickens. Make sure it is not too watery and you know that you have put in too much cheese when it becomes lumpy. This might be a little bit tricky and requires some eyeballing, but you will get the hang of it after a while.
Once the spaghetti is done, drain it and set it aside. Using the same pan/pot, cook the bacon and onions until it turns light brown. You will know its ready when you smell the 'oh so yummy' smell of the bacon that will give you a heart attack by just inhaling the aroma.
There you go. A plate of hot spaghetti carbonara. No frills, no gourmet ingredients. Just a simple traditional recipe handed down by word of mouth through the generations. Hope you like it!