Cooking sugar is a fairly simple concept: boil down sugar crystals until the crystals can reform into a liquefied, chewy, or crunchy texture. The difficulty is in getting the correct temperature for this new form without burning the sugar.
While you certainly can (and should) use a candy thermometer to monitor your recipe, the precise nature of sugar cooking makes getting the correct texture difficult when deciphering a thermometer. Furthermore, most candy thermometers are difficult to hold in the correct position (very center of the recipe without touching the pan) while constantly stirring a boiling pot of syrup.
Here’s what I recommend:
(1) Have ready a cup of icy water.
(2) Use your thermometer to get to the approximate temperature of your desired sugar form.
(3) Once you get within 5 degrees of your desired state, spoon small threads of your recipe into the ice water.
(4) Allow briefly to cool, and then fish out the thread and see where you’re at! Continue testing until you reach your desired state.
Thread will not ball when rolled.
Soft ball (240°F)
Able to be rolled into a ball with a soft exterior.
Hard ball (250-260°F)
Hard outer shell, but still able to be molded if firmly pressed.
Soft crack (270-290°F)
Hard threads. Still somewhat pliable but will crack if bent.
Hard crack (300-310°F)
Hard threads. Not pliable and will crack if pressed, bent, or otherwise manipulated.