Short answer
Starting from the right, multiply the body digits alternately by 3 and 1, add them and choose the digit that completes the next multiple of ten.
Start beside the check digit
The existing rightmost digit is excluded from the calculation.
Alternate weights 3 and 1
The rightmost body digit receives weight 3, the next 1, then 3 again.
Complete the next ten
Use the remainder modulo 10; the amount needed to reach ten is the check digit.
A checksum is not proof of assignment
This catches structural errors but cannot establish who issued the number.
