Step 1: Divide the number into separate digits:
39842 => 3 9 8 4 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 = 6
9 to 8 = 1
8 to 4 = 4
4 to 2 = 2
2 to 3 = 1
Step 3: Combine the filters to a string:
6 1 4 2 1 = 61421