There are few tips to keep in mind BEFORE you begin. First, sugar is finicky to work with, do NOT be tempted to stir it once directed NOT to in the recipe or you'll end up with a cloudy mess.
Use a clean METAL spoon, and do NOT touch the syrup until it has cooled completely.
If there is a chance that your candy thermometer is not accurate, here's a simple test you can do before you start cooking with it.
5 1/3 C. Granulated Cane Sugar
2 C. water
1 tsp cream of tartar
Dash of salt
Combine all 4 ingredients in heavy saucepan (with a candy thermometer clipped to the side!) over medium heat; Stirring just enough to moisten the sugar.
Do not stir it again, stirring causes the sugar to crystallize.
Allow the mixture to come to a boil, once it has you'll need do dip a clean pastry brush in water and brush down the sides to remove any sugar crystals that may have formed (and will consequently ruin the jar if they remain!).
Continue boiling the syrup, cook until it reaches 235-240 F on candy thermometer, then immediately remove the pan from the heat. Let the syrup set until it is completely cooled, about 2 hours.
Do not disturb the pot during this time.
This syrup can be stored for 3 months in the pantry without issue. Simply transfer it to a clean mason jar (we use half-pint jars- which are 1 cup) for easy measuring for our favorite recipes.
If the syrup is too thick, remove the cover from the jar and warm it in the microwave for 30-45 seconds.
Makes about 1 Quart
When should you use your own homemade and when should you just buy the real thing? Check out this page that explains it all.