I don't know about tutorials for C++ programmers learning C, mostly they're the other way around. If you're already fairly comfortable with C++ just go for a straight C tutorial and you should do just fine.
Do keep in mind that C has changed in recent years. The C standard was updated in 1999, introducing some new features. I would imagine that most tutorials on the web are based on C89 rather than C99. I don't know which you're required to use for your task.
If using older, more traditional C I would suggest you get hold of a copy of Kernighan & Ritchie's C Programming Language second edition. It's a good intro to C (still selling well), and with a C++ background I would expect you to be able to go through it pretty quickly, and soon pick up C I/O and other differences with C++.