You just need to let the impurities burn off. There shouldn't be any smoke, or very, very little at most (impurities no longer present), when the charcoals are red/white hot. At this point this is when the charcoals are producing copious amounts of invisible CO gas. That's when you place them into whatever enclosure you have and wait more time to allow the CO level to build-up in your enclosure.
Now, here's the disclaimer: the higher the CO level gets, the quicker, and more painless, the death. Lower levels of CO may still kill someone, but it will take more time. The longer someone is exposed to CO and not rendered unconscious, the more symptoms they may consciously experience, some of which may be very uncomfortable, even painful. At low CO levels, or even at high levels, whereby someone isn't "allowed" to be exposed long enough (they get "saved" before death), irreparable neurological damage can occur - basically, if someone ends up surviving the attempt, they could end up a vegetable for life.
There's a formula for calculating how much charcoal to use for a given volume of space. I suggest you do more research on this method before attempting. Ideally, someone would want to achieve a CO concentration of 10,000 PPM (Parts Per Million), or more, in their enclosure. How do you know if the CO level is that high? They make an analyzer that measures CO concentration, but you need one that measures that high. It can be successfully done without it, and it has many times, but it's more risky. That's why it's important to do your homework on this method.