Add the balsamic vinegar to a small saucier or saucepan. Cook it over medium-high heat until it begins to boil, stirring often.
Once it reaches a boil, turn the heat to medium-low and simmer for 10-20 minutes, stirring often. You will know it is ready when it has reduced by half and starts to coat a spoon.
Remove the pot from the heat and let it cool. The glaze will thicken as it cools.
Notes
Store in the fridge in a sealed jar for up to 2 weeks.
The time it takes to reduce will depend on the quality of vinegar used.