What IRS forms need to be signed by an independent contractor?

All income tax forms and any other business forms that have to be submitted to the IRS quarterly, etc to take care of the self employed business operation correctly and by the rules.

For instructions and forms go to the IRS.gov web site and use the search box for publication 334 Tax Guide for Small Business a very good place to start with examples

Publication 463 Travel, Entertainment, Gift, and Car Expenses

Use the search box at the IRS gov web site for Small Business and Self-Employed Tax Center

Filing Season Central is your one stop assistance center for filing your business returns. This includes Highlights of Tax Law Changes, Tax Tips, and more.