public static void SetProcessAffinityMask(int affinity, string processName = null)
    {
        Process myProcess = GetProcessByName(ref processName);

        Console.WriteLine("Process {0} Old Affinity Mask is : {1}", processName, FormatAffinity((int)myProcess.ProcessorAffinity));

        myProcess.ProcessorAffinity = new IntPtr(affinity);
        Console.WriteLine("Process {0} New Affinity Mask is : {1}", processName, FormatAffinity((int)myProcess.ProcessorAffinity));
    }

Example of usage :

private static void Main(string[] args)
{
    int newAffinity = Convert.ToInt32("10101010", 2);
    SetProcessAffinityMask(newAffinity);

    Console.ReadKey();
}
// Output :
// Process Test.vshost Old Affinity Mask is : 11111111
// Process Test.vshost New Affinity Mask is : 10101010