First of all let me tell you that cash book is balanced on daily basis and the cash balance shown by cash book is verified with the actual cash lying with the cashier. So this mistake normally comes to notice on the day it is committed. If debit side of cash book is undercast it means cash book is showing less amount as compared to what is actually lying with the cashier. There are two ways of rectify this mistake:
1. Without putting a journal entry. First of all on the debit side of cash book put the date of making rectification in the date column and after that under the particulars column write like this 'Amount undercast on this date' and put the amount by which the cash book is undercast in the amount column. It is done.
2. Second alternative is you put the following journal entry:
Cash A/c Dr
To Suspense A/c