You will need a medium saucepan and a bowl that will fit comfortably on top of the saucepan but not touch the water. I use a stainless steel bowl on top of the saucepan, but a glass bowl would also be suitable.
Fill the saucepan 1/3 full with water, and heat the water to a gentle simmer.
Place the bowl on top of the saucepan, and add the sugar, butter, lime juice, lime zest and salt to the bowl.
Gently stir until the butter is melted and the sugar has dissolved.
Crack the eggs into a separate bowl, and whisk the eggs vigorously until the mixture is frothy and no visible egg white can be seen in the mixture.
Pour the frothy eggs into the lime juice mixture, and stir gently to combine.
Continue to gently stir the mixture for about 15 minutes, or until the mixture has thickened and coats the back of a spoon.
If the mixture becomes quite hot, or the water starts boiling, remove the bowl from the top of the saucepan, and allow to cool slightly before proceeding. If the mixture gets too hot, the eggs will curdle, which will ruin the curd.
Once the mixture has thickened, either bottle it using the instructions below, or allow to cool and use within 3 days.