Okay to answer your questions:
You place a Draconic core on a standard spawner, it will turn it into an empty draconic spawner. You can then pick it up with a DE pickaxe and move it to where you want.
Next up is to get either a DE sword (wyvern or full draconic) or enchant a vanilla one with reaper. You can also enchant the DE swords with reaper to improve your chances. Then go hunting for the soul you want. This will take time as an unmodified game it has by default a 0.1% chance to drop a soul.
(At this point I will stress this: The DE swords can get Mob souls by default without the reaper enchant. The Reaper chant can be applied to most non-DE swords to give them the capability to get souls at the same rate as the DE swords. Applying the reaper enchant to a DE sword however increases their chance to get souls than default)
Next you take your soul and right click the DE spawner. It will now work the same as a vanilla mob spawner with all the same traits (player proximity, light level, etc. The DE spawner however can be upgraded with various items for various effects:
Wyvern core: increases spawn rate as well as count and allows redstone control.
Awakened core: Further increases spawn rate and count.
Nether Star: removes player proximity requirement. you can be as far away as you can still chunk load and it will still spawn.
Notch Apple: Ignore light level.