First, iterate through each index `i` in the array `nums` and count the number of set bits (1s) in its binary representation. Then, sum the elements `nums[i]` where the count of set bits equals `k`.

To count the set bits in an index `i`, you can use a loop to check each bit position or utilize built-in functions like `__builtin_popcount(i)` in C++ or `i.toString(2).split('1').length - 1` in JavaScript. Ensure the count matches `k` before adding `nums[i]` to the sum.

Optimize the set bit counting by using bitwise operations. For example, use `i & (i - 1)` to clear the least significant set bit and count iterations until `i` becomes zero. This reduces the time complexity compared to checking each bit individually.