Lets look at the factors involved if we are to try this...

World: We would have to operate in this world (Assiah) because it is the only known world they reside in. By that I do not mean that it is just this Earth, demons have no physical boundaries so it is quite possible to use other planets as a crash pad where there is nothing and no one (human) to verify or monitor them. Practical Kabbalah in Assiah still functions as in the other worlds but is more 'quick and dirty' in comparison to the others. The applicable sephira is Malkuth in Assiah.

Demon: We would have to find a more than willing participant, certainly possible but there is an 'order of things' even in the demonic realm. Trying to get a 'left hand man' on Belial to jump ship may not be all that practical, it is certainly possible but will take time and quite a level of difficulty and indeed risk - not only to you but also the people you hold dear. Much like a criminal with a personal beef against you, if they cant get directly at you theyll go for your kids.

Atheist: Though one may be tempted to go after say.... Dawkins or Frank, we would have to be in physical proximity to their residence and maybe even them. We have to know everything possible about them such as birthday etc. There can be no randomness in selecting a target, randomness in application of such an operation will result in randomness of results.

Assistance: It is always good to have assistance from the good guys, although it might be possible to do this by yourself you will have to seriously look at doing magic ritual and by default will be dealing with an angel somewhere along the line. If you are looking at doing practical Kabbalah in this you will be dealing with Adonai Ha-Aretz in Malkuth but this is interchangable with Christ Jesus. But you can opt to consult angels and saints or even the Virgin Mary, but until you get a demon and atheist you really cannot come up with a hardcore game plan.

Courage: Very important! You will be going on a journey better than crank and sex, you are basically betting everything on a turn of a card. But if you have a good game plan and win youll be going "OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOHHH! WHAAAAAT A RUSHHHHHHH!" (from the Road Warriors wrestling intro)


So where do you get a demon from? Well in your case I would recommend a rather low ranking one, such servant demons can be found in books such as Abramelin and indeed there are 'demon dictionaries' from earlier years and not the new age fluff that passes as occult knowledge these days. It is said that every nation has a demon assigned to them, if you can find out which one you have then you can try and get a list of servants. But are you planning to do this solo or with others? If it is with others do you all live in the same country? And even then what level of occult knowledge do you all have and who has the most experience and what country do they live in and are they willing to take a lead role? You could just decide to pick a demon at random from a dictionary but you must be able to research everything about them.

You need to know.....
All the names they are known by and especially unique names by which no other demons are known.
Their title (if they are known to have one)
If they are used directly in any magical operation, if so what branch of magic it is.
Who is their supervisor and who is the supervisors supervisor.
Their sigil.
What their responsibilities and abilities are.
Are they under any planetary constraints.
Do they have a zodiac sign.
Are they able to function to mechanical hours rather than planetary ones.

From this you are definitely identifying a single entity, you are taking an interest in them and they will most likely take an interest in you and this is why you need courage and not shit your pants if they show up at your place. However you cant go abracadabra at the moment, you need an indirect form of movement and this is done by performing a system of prayer for the redemption of the demon. This can be done in a novena schedule or something along those lines, but remember you are not only asking for the demon to repent and seek forgiveness. You are also asking God to be forgiving and accept the repentness and both items must be dealt with in any prayer that you put forward. Until you have set this into motion then selecting an atheist (though you should have a list of candidates) or putting any occult acts into motion will be pointless.

But also take note, you must reconcile to God yourself and seek to spiritually strengthen yourself through prayer/spiritual exercise. Much like a boxer will throw hundreds of punches during sparring so it will be effortless to throw the punch he needs in a match.