I too use a basic Excell spread sheet.
List date... customer.. amount invoiced.. GST.. total with GST.
Add columns at end of every quarter and Bobs your uncle.
I haven't found an easy way to do a BAS yet apart from just handing over a ton of receipts to someone else to do, but it all gets logged in a different spread sheet and my accountant gets both at tax time.
Bottom line.. ANY paperwork is  painful if like me you were not born to do any type of office work.
I bloody hate it with a passion