Complete plans provide you with a different set of challenges because following someone else's plans is never as straightforward as following your own. Following a good set of planning will stretch your own class control skills - its a little like driving someone else's car. You know how to do it, yet it feels a bit clumsy.
The key thing here is to read through the plans you have been given carefully to make sure that they make sense to you. Do some mental rehearsal as to how the lesson will go. Get familiar with the resources. If you have time, practise the skills you are teaching to ensure you are familiar with it and can answer any questions that might come up.
When the day is over you should spend a little time providing assessment information to the class teacher in as much detail as you can - did any groups struggle? With what? Were the top group stretched enough? What common misconceptions were experienced?
Tip: try saying something like this to the class:
"Miss .... has asked that you all manage to do .... (set a target for output). I'm here to help, if you need it. Do your best and show Miss.... what you can do even when she is not here. I'm sure you can impress her."