It should be easy enough to implement. I grabbed this description from KSU,
https://people.cs.ksu.edu/~schmidt/115/ch5.html
They also include an example calculation so that it is easier to understand, just search the page for "Fletcher's checksum"
There is a more complicated explanation on
Wikipedia, which also has implementations in C
If you need help with specific spots you can ask. but this feels homeworky so regardless of whether it actually is or not, I won't just offer complete code.