Procedures in accomplishing the BIR Form No. 1600
Hint: Use tab to move to the next enterable field
Indicate the period for Item 1 by supplying the for the month of field in MM/YYYY format. 
Number of attachment in Item 3 will automatically increment as you attach a file.
Indicate if there are any taxes withheld in Item 4. Selecting Yes will enable fields under Computations of Tax.
Indicate category of withholding agent in Item 12. This is a mandatory field.
Change the default data under Item 13 if you are availing of Tax Relief. If Yes is specified, the "If yes please specify" drop down box will become mandatory.

Supply applicable Tax Base under Tax Base column of Part II computation of tax after selection of ATC by clicking the hyperlink. Tax
Required to be Withheld will be automatically computed based on the
rate specified on the form.

Item 15 will be retrieved from the DB if you are filing an amendment. This field will remain editable, change the values as necessary to reflect to correct info.
Fill up Schedule II and supply necessary fields as needed. ATC can be supplied by clicking on the hyperlink and selecting from the list. Complete all data for each row by filling the Payee details and value of
Amount. Tax Required to be withheld column will automatically compute after completion of required columns .
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.

Grayed out fields will be automatically computed upon either completing required fields or clicking on validate button.
Press or Click the Validate Button.
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 12.
Review the data you have just encoded then click on verify to have an automated validation process. The taxpayer will be prompted if any problems were encountered. Otherwise, a successful validation will enable the Submit button. 
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.
A reference number will be given by the system for filed tax return.