Procedures in accomplishing the BIR Form No. 1601C
Steps
Hint: Use tab to move to the next enterable field
1
Indicate the year when your monthly remittance was earned. In this case this field is defaulted to the current month and year (Item 1 "For the month").
2
Item 2 Amended return will be disabled and automatically reflects if the Form you are trying to file has been filed previously (Based on Item 1)
3
Indicate if there are taxes withheld during the previous year. (Item 4 "Any Taxes Withheld?") Defaulted value is Yes, selecting no will disable fields pertaining to computation of tax.
4
Select from the option button the category of the withholding agent. (Item 12 "Category of Withholding Agent")
5
Supply values for the Amount of Compensation on items 15 and 16, Tax Required to be withheld item 18 and Taxes Remitted Previously Filed item 21 (only applicable for amended return, the value has been automatically retrieved, change as necessary). The Taxable Compensation item 17, Tax Required to be Withheld for Remittance item 20 and Tax Still Due/(Overremittance) item 22 are automatically computed by the system.
6
Adjustments, item 19 will be filled up by data upon completion of Section A, Adjustment of Taxes Withheld on Compensation for Previous Months.
7
Penalties, Item 23A, 23B, 23C, will be computed automatically by the system.
8
Choose between Monthly and Yearly radio buttons to indicate the type of data being supplied. If Monthly, Input values for the Adjustment of taxes Withheld on Compensation for previous months, the date paid, bank validation ROR No., bank code, tax paid (excluding penalties), should be tax due for the month, adjustments from current year, from year-end adjustment of the immediately preceding year. If Yearly, some of the fields will be automatically disabled, fill up data as needed.
9
Total, Item 25, is automatically computed and the value of which is passed to Item 19.
10
The taxpayer can now attach supplementary documents by clicking on the Add Attachment button. A separate window will be displayed to facilitate the process. Attachments are done on a per file basis. 

Please note that the system currently accept files of the following format only: *.zip, *.xls, *.dat, *.csv. The taxpayer will be prompted if the file is not a valid format.

We do suggest that you upload your files in zip format to shorten the process.

11
Press or Click the Validate Button.
12
The system will prompt you if there is any problem with the supplied information, and correct this accordingly. After the necessary correction, you may again validate the return form by repeating Step 13.
13
If you want to edit some input fields you may do this by clicking the Edit Button located on the bottom of the page. Or, if you are already satisfied with the result and all the information is correct you may already submit this by clicking the Submit Button beside the Edit Button.
14
A reference number will be given by the system for filed tax return.