how do I enter my own 1099s?

The instructions are different depending on which type of 1099 you have.  But when you choose to add the 1099, look for  "Enter a different way." That is where you can manually enter your data.

Here are the instructions for one of the most common 1099s, 1099-INT: 

Open TurboTax Online and navigate to Wages & Income > Investments and Savings (1099-B, 1099-INT, 1099-DIV, 1099-K, Crypto) > Add/Edit

1. Choose the button Add Investments
2. Under Let's import, choose Enter a different way.
3. Choose Interest, continue
4. Choose Type it in myself
5. Enter FEIN from payer, payer's name, and amount of interest. Choose recipient's name from dropdown box.

Which Form 1099's do you need to enter?  There are almost two dozen different Form 1099's.