Put the jam into a small saucepan. Add 1 tsp water to the pan and heat gently over a low heat until the jam has melted, without letting it boil. Avoid stirring the jam too much or too vigorously or you will incorporate air bubbles that can cause it to cloud.
Once melted, if the jam is still very thick add 2–3 tsp warm water to loosen it.
Adjust the consistency with a little more warm water until the glaze coats a pastry brush evenly and is the consistency of runny honey. Some jam will not need any water added and some might need more, but take care as too much water will thin the jam too much and it will not adhere properly.
As you use the glaze, keep checking the consistency and adding water as necessary, as it will thicken as it cools.