Make Transaction/Journal Voucher Form in Excel for Accounting

Dear all,

In the field of finance / accounting, journals and transaction authorization vouchers are needed. 
I once got the authorization voucher form format. But I need to change the format according to the conditions of the office where I work. 
However, I can not see the formula used in the voucher form. 
The voucher needs to be changed to a month (drop down), then also "Voucher No" (voucher number).

The voucher entry links to another sheet.

Can anyone help?