Click "Insert" at the top and then click "Drawing."
In the "Drawing" popup, hit the "Text box" option which you can find between "Shape" and "Image." Now, draw a text box on the "Drawing" popup.
Enter the text you prefer within the text box you've drawn. If you wish, you can customize the text box using the options at the top which you find after the "Image" option. If you need, you can resize the text box by dragging its resizing handle.
Once you are okay with the text box, click on the "Save & Close" button at the top.
That's all. Now, you can see the text box in the place you've preferred in the document.
You can edit the text box at any time you want in Google Docs and here's how you can do that - Select the text box you want to edit and then click on "Edit" link. Now, you can see the "Drawing" popup where you can make the changes you want.