I need help in creating a 'checksum' from a string using the following conventions:

1) Add the 1st byte of the string to a checksum byte
2) Clear Bit 7 of the above result
3) Set Bit 6 of the result
4) Add this result to the next byte of the string
5) Repeat steps 2-4 until last byte of string is read.

Please Help! I don't know how to get started?!