It will become second nature soon enough. Since warlocks use damage over time spells, the target will die without you having to keep casting. This allows you, before it does die to cast Drain soul. Continue casting this, because if it is being channeled when the target dies, you will create a soulshard. It will automatically be deposited into your bags, and you don't have to do anything with it other than to make sure you have at least one readily available to summon your demons(besides imp) and summon other characters to you.