[erlang-questions] checksum calculation

[Gamoto]

I have a string of bytes and would like to compute the checksum which is the xor of all bytes.

Let S the string, N the current byte

checksum( <<N,Rest>>,Csum) ->
  checksum(Rest,Csum bxor N);

The problems are: I don't know how to start (checksum(<<S>>) ???) and how to finish !
First of all, is it the correct approach ?
(I suppose I rather should make Csum Exclusive-OR N modulo 8     ???)