My father is dead and I must file his final return. I am the Executor. Obviously he cannot sign so how is that handled?

On a paper filed return you would sign in his space and indicate a title such as Executor.  You can also e-file the return. See https://turbotax.intuit.com/tax-tools/tax-tips/Family/Death-in-the-Family/INF12090.html  regarding using TurboTax for a death in the family. E-filing is discussed here: https://ttlc.intuit.com/questions/1899441