Once you have a Soul Infuser, make some Soul Jars. To make them, right-click/interact on the Soul Infuser Core using a Glass Bottle, and then a Soul Silver Ingot. This will give you an empty Soul Jar.
If you venture out into a Soul Sand Valley in the Nether, you will find one of these flying creatures, which you can catch in your Soul Jar by right-clicking/interacting on them. They are used as the basis for creating spawn eggs.
To make a spawn egg, place a Soul Spark on each Pedestal on the Soul Infuser. Soul Sparks are rare drops from most hostile mobs. Right-click/interact on the Soul Infuser Core with the Soul Jar, and then right-click/interact using a matching amber (a new ore) or mandrake (a new plant). If you did it correctly, you will get a random spawn egg.