Lisa just a basic layout like below is fine, use a table or spread sheet.
Customer Name
Statement  09/01/2008
Date   Invoice No         Amount                Paid to Date   Balance
01/11/07   1559           1145.62                  0.00                   1145.62
15/11/07   1601           21.68                  0.00                   1167.30
01/12/07   1632           916.50                  0.00                   2083.80
01/12/07   1633           540.50                  0.00                   2624.30
01/12/07   1642 Credit   -540.50                    540.50                   2083.80
14/12/07   1661           15.86                  0.00                   2099.66
21/12/07   1691           28.73                  0.00                   2128.39
01/01/08   1686           916.48                   0.00                   3044.87
07/01/08   1712           15.86                  0.00                   3060.73
08/01/08   1714          13.04                  0.00                   3073.77                        
Total Due to date   2083.80   (obviously you would line it up better!  

)   
The total due would be on your terms -  this one is 30 day term, you could do an aged system where the total is stating how much is 3 months +  2 months + 1 month + current month but really there is no need to complicate matters.  Or like me get a system that does it for you!
Fox