Step 1: Divide the number into separate digits:
6023 => 6 0 2 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 0 = 6
0 to 2 = 2
2 to 3 = 1
3 to 6 = 3
Step 3: Combine the filters to a string:
6 2 1 3 = 6213