A common beginner mistake is that people try to hit the whip with alot of arm force to the ground. This is not the right way to use a stock whip. Instead start by holding the handle of the whip correctly in your dominant hand, with your feet shoulder widths apart. Once you bring the whip up smoothly, then just let your arm fall instead of jerking it. Let the weight of your arm do the work instead of your muscles.
Another common mistake is that people advance too quickly to advanced cracking tricks. Make sure you master the basics first. Simply start by practicing the forward crack by bringing the whip to 12 o clock position right above you and then cracking it in front of you with your elbow in a locked position and letting your arm do the work. Looking at training videos of other people performing cracks may also help as you can see what it is suppose to look like.