Anonymously and online. Or, you can leave QR codes around.
Dont forget what Satan said in the Al-jilwah.
"I have not taught these teachings, nor do they proceed from me. Do not mention my name nor my attributes, lest ye regret it; for ye do not know what those who are without may do. "
Also don't forget to take safety into account. Please check the internet activism forum for further instruction and, please, don't waste your time with hard core brain washed christians. It isn't worth it when even after hours it wont change their mind.