I usually mix in a 5 gal bucket the night before I use it. Mostly because it takes a while for it to cool.
Dont boil the syrup, or it will solidify like crystallized honey.
I usually put the amount of sugar I want into the bucket, and in a big stailess pot bring the water to near boiling.. that point where it hisses and bubbles are starting, but its not really boiling yet, then I dump that into my 5 gal bucket on top of the sugar, then I use a wine stirrer/degasser like this one;
In my black and decker battery powered drill to stir until it begins to clear. Everything dissolves well and I dont have crystalization problems.. or at least, have not yet.... I let the bucket settle and cool overnight.