There are a huge number of ways to do this, but I'd do it in two stages:
1) Write a method to convert a binary string to an integer.
2) Write a method to convert an integer to Octal.
Now, there are .NET methods that will do both of those, but I assume this is your homework? So you have to write them yourself?
So start by writing "placeholder" methods which use the .NET framework methods to check your code:
] will handle binary string to integer
] will convert an integer to a string in the specified base.
Then, you can just write your own code for each method and make sure you get the same results!