Data can be submitted in one of 3 ways: manually entering the data on the website, uploading a .csv file (basically an excel document with no formatting saved in .csv format), and transmitting data through and application programming interface.
According to the rule, companies with 250 or more employees are required to submit form 300A by December 1, 2017 with 2016 information. For their 2017 information, those same companies would be required to submit forms 300A, 300, and 301.
For companies that have 20-249 employees and that are in high risk industries like construction, only form 300A will need to be filled out this year and in subsequent years.