I've made it before.
A couple tips
1. Make sure all your containers and pots, etc are really clean
2. Don't overheat the milk; too hot and it will kill the culture
3. Don't go crazy stirring the culture yogurt in, stir/mix it in gently and just enough to combine into the milk
4. Don't bump or jostle the container(s), just let them go
5. Don't be too disappointed if the finished product isn't as thick as store-bought, much of that has thickeners in it. You can strain it through cheesecloth if you want it thicker