How do I begin inputting information on a second person's tax return after I've finished inputting my own?

[kengalli]

we are now trying to complete my father-in-law's tax return for 2018.

[Ashby]

You have to create a second, completely separate account for your father-in-law. Then, you can begin his return.

[VolvoGirl]

Are you using the Online version?  The Online version is only good for 1 return. But you can use the same email address for 5 accounts.  To do another return you have to set up a new account and new user name and pay any fees again. 

https://ttlc.intuit.com/replies/3288574