If I could give one piece of advice to someone making up a fake religion, it would be "don't do it". But if I was the lying type and actually wanted a fake religion to succeed, my one piece of advice would instead be "don't give authority to the Bible." See, the Bible is quite a menace to false religions, being the inspired word of God and all. If authority is given to it, it becomes perfectly legitimate to use it to test the false religion.
For example, Islam claims that Jesus was not the Son of God. That wouldn't be too much of a problem for Islam if Muhammad never gave authority to the Bible. Unfortunately for Muslims, he did. He appealed repeatedly to the Bible and those who followed it (Qur'an 3:55, 71; 5:47, 50, 71; 10:94; 29:46; 42:13; 61:14). Because Muhammad gave so much authority to the Bible, the Qur'an cannot be true unless the Bible is also true. If the Bible is not true, the Qur'an is not true. But the Bible disagrees firmly with the Qur'an on several issues. Therefore, if the Bible is true, the Qur'an is false.
In 100% of all possible scenarios, the Qur'an is false. Therefore, the biggest mistake Muhammad made was giving authority to the Bible. While I have used Islam as my example, it isn't the only religion to make this mistake. Many religions, especially pseudoChristian religions, also give sufficient authority to the Bible to thoroughly refute themselves. Creators of false religions would be wise to follow the wisdom of Proverbs 30:6. Do not add to His words, lest He rebuke you, and you be found a liar.