"How are they doing that?"
Translation:Comment font-ils cela ?
Faisant: Present participle (as extracted from http://french.about.com/od/grammar/a/presentparticiple.htm )
"1.The French present participle can never be used to talk about what someone is doing. The construction "je suis mangeant" (the literal translation of "I am eating") simply does not exist in French - you must use the present tense: je mange. To emphasize the ongoing nature of an activity, you can use the French expression être en train de: je suis en train de manger - "I'm eating (right now)."
Thus the present tense should be used: Comment font-elles ça?
For future reference consult online guides and websites, when uncertain about a question.
I hope that is useful. Kind regards.