Step 1: Divide the number into separate digits:
81684 => 8 1 6 8 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 1 = 7
1 to 6 = 5
6 to 8 = 2
8 to 4 = 4
4 to 8 = 4
Step 3: Combine the filters to a string:
7 5 2 4 4 = 75244