Heat a bit up in a saucepan (a small bowl in microwave is even better, less fuss to clean) with a bit of water or lemon juice to thin it. You’ll know when it looks thin enough to brush on. Then brush it onto whatever you are applying it to. Whether you sieve it or not first is up to you. Some think it looks a bit more appealing with bits of fruit in the glaze.
You will come across a food writer here and there who will say that apricot jam isn’t good enough, it has to be apricot preserves, as the jam contains too much sugar which upsets the balance of the required acidity in the glaze to act as a foil to what you are glazing.