Use a ladle and scoop out the lemon mixture into a sieve which is over a large measuring cup or pitcher.
Continue doing this until all the lemons and juice are out of the pot.
While scooping the lemons out don't forget to squeeze out extra juice while the lemons are in the sieve.
Add 5 cups of cold water and allow to chill in the fridge.
Notes
I have cooked the mashed lemons and juice on the stove until it was starting to simmer. This step is not necessary, but it does help with bring out the flavor a bit more. This step is optional, but it doesn't take much longer to do.