There are three ways to add the sugar:
When making your lye-water solution, add the sugar to your water before you add your lye to the water. Make sure it's completely dissolved before adding the lye. (You can also add salt to the water at this point too.) This is the easiest method, though sometimes the heat of the lye solution will begin to caramelize the sugar a bit, and turn the sugar water a bit of a beige color. Not quite like the orange color you get with goat's milk soap, but for the same reason. This is just an aesthetic consideration. It doesn't affect the soap at all.
Take a bit (just a few ounces) of the water you’ve weighed to make your lye solution. Add ½ - 1 tsp. of sugar per pound of oils you are using* to the water, and stir until it’s completely dissolved. Sometimes it helps to warm the water first. Add this sugar-water solution in at trace with your other additives, but before your fragrance oil.
Make a “simple syrup” ahead of time by taking two cups of sugar and one cup of water and slowly heating them on the stove. Stir gently until all of the sugar has dissolved. Let it cool and pour it into a bottle to use when you’re making soap. Add ½ to 1 tsp. of the syrup per pound of oils you are using* to your soap, at trace, with your other additives, but before your fragrance oil.
* Sugar Measurement
"1/2 to 1 tsp. of sugar per pound of oils you are using" means that for each 16 oz. of oils (just the oils) you add that much sugar. If there is 32 oz. of oils, add 1 to 2 tsp of sugar. If there is 48 oz. of oils, add 1 1/2 to 3 tsp. sugar.
Note: Some people have noticed that adding sugar to their soap makes it heat up more in the gel stage. The same thing happens when milk is added - it's the sugars in the milk that is jump starting the gel process. Be careful if you are adding sugar and adding milk. Your soap can get quite hot!