Step 1: Divide the number into separate digits:
39002 => 3 9 0 0 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 = 6
9 to 0 = 9
0 to 0 = 0
0 to 2 = 2
2 to 3 = 1
Step 3: Combine the filters to a string:
6 9 0 2 1 = 69021