Method : 

• Juice 2 - 4 fresh Lemons , you are going to need 200 ml of lemon juice.
• Put the caster sugar into a saucepan on a low heat.
• Add the Lemon juice to the sugar and slowly bring to the boil.
• Reduce the heat to a low simmer.
• Let the mixture simmer for around 10 minutes or until the sugar has completely dissolved  and the syrup has thickened slightly.
• Add the Lemon Zest.
• Decant the Syrup into a jar or bowl and allow to cool.

If you want a thinner syrup then simmer for less time , about 5 minutes. The longer you simmer this the thicker it will become....