Step 1) Go to Master > Other Master > Voucher Prefix Setup
Step 2) Click on Add (Alt + A)

Step 3) After that select Transaction type like sale , Purchase , Cash , Bank and Journal Entry
Step 4) Add Prefix in box you can use 5 alphabetical only.
Step 5) After that add start from the Voucher add.
Step 6) Select default Book name , Format , Memo type and invoice type.
Step 7) After that Press Save (F2)