How do I report undocumented income? I usually receive a 1099, but I didn't this year.

You have to report all income earned during the year on your tax return. Your side job would be reported on a Schedule C (self-employment income). 

 

For tax purposes, self-employment income includes any part-time businesses or "side work" performed in which you are in business for yourself, rather than another person. You are also considered self-employed if you engage in business-like activities where you intend to make a profit. 

 

You are able to file a Schedule C without a payer's employer identification number (EIN) and not associate it with a 1099. When you are inputting information for your Schedule C, click on the option Other self-employed income under Common Income. This includes cash and checks received and income not reported on any particular 1099 form. 

 

Please follow these steps in TurboTax to start your Schedule C:

1. Open your return.
2. Search schedule c with the magnifying glass tool on the top of the page.
3. Go to the Jump to schedule c link in the search results. 
4. Answer Yes to the page titled, Did you have any self-employment income or expenses?
5. Follow the on-screen instructions.
6. On the page titled Let's enter the income for your work, click on Other self-employed income under Common Income.
7. Enter your type of income (cash/check) and the amount.
8. Follow the on-screen instructions.

My wife and I are musicians who perform and teach private students.  We have some 1099s but much of the income is not documents by 1099 or W-2.  

A bigger complication is that I have been trying to add a Schedule C to report my undocumented income, and it seems to only allow for one Schedule C, which is my wife's.  In the past I was able to include separate a Schedule C for her and for me.  Is there a way to add another Schedule C?

Yes, you can add another Schedule C,

To enter your Schedule C or an additional Schedule C follow these steps:

1. Open or continue your return
2. Search for schedule c and click the Jump to link in the search results
3. Answer Yes to Did you have any self-employment income or expenses?
4. If you've already entered self-employment work and need to enter more, select Add another line of work
5. Follow the on-screen instructions to add your self-employment information.

@astreder