Add sugar and water to a deep saucepan. Stir and bring the mixture to a boil, then reduce heat to a simmer and skim off any foam.
Add the lemon juice and continue to simmer. Add a few strips of lemon zest if using.
Let the mixture simmer for about 35-40 minutes. Keep an eye on it and adjust heat as needed.
To check if the syrup is ready, dip a chopstick in the syrup briefly, then let it drop on a plate. If the droplet spreads slightly on the plate but still forms a round shape, the syrup is ready. You can see this test in the video in the post.
Another way to check is by measuring the weight of the syrup. You should have about 1.2kg (about 2.65lbs) of syrup. A little more or less is fine but don't go over 1.3kg (2.85lbs) or under 1.15kg (2.5lbs).
The finished syrup will have a rich amber color with a runny texture that is thinner than honey and will thicken over time. Transfer it to a clean container and let it cool completely. Then cover and store in a dry place at room temperature.