Luhn algorithm(i think you know that) calculator:

If number is in an even, add it to the sum.

else If number is an odd, times 2 and add the digits of the resulting product to sum.

If the whole number is valid, the final sum will have an 0 as end.

if number is 6411.

The first digit, 6, times 2, =12, so 1 + 2 = 3, add to sum.

The second digit, 4, add to sum.

The third digit, 1, times 2, =2, so 2 is add to sum.

The fourth digit, 1, times 2, =2, so 2 is add to sum.

So the checksum is 3 + 4 + 2 + 1 = 10. valid.