Updating user details from file

Hi,

I am trying to update the telephone numbers of users in my client's organistation en masse in Active Directory. I believe I should be able to use the dsmod command to do this and, although a few places have said it can be done easily from a file, have not actually shown HOW to do it. I don't know what switch and/or command I should use in dsmod to incorporate a file, how the file itself should be structured (.txt, .csv, column names, etc?), or whether dsquery should be used first...

Can anyone point me in the right direction here??