You say you don't have Office but you have Excel and Word. These are
important components of Office so you do (or did) have Office.
In its simplest form the answer to your question is to have four columns in
Excel
A- Date
The date each CS payment is due or received
B- CS due
The amount of CS due
C- CS received
The amount of CS received
D- Over / under paid
A running balance of over / underpayment. This can be done with the formula
=SUM($B$2:B2)-SUM($C2:C2)
copied down to each row