It is as simple as going to the Insert Menu
If you want to add one you've already created you can choose from Drive and window will appear showing you all of your most recent Google Drawings.
Select the drawing you'd like to add in to your Google Doc, press Select and then you will be prompted to choose Link to Source or Insert Unlinked.
The difference between Linked and Unlinked is that Unlinked treats the Google Drawing the same as adding a image. If you make changes to an unlinked inserted Google Drawing you will have to re-upload the new drawing after changes have been made, vs just clicking update.
The other option is to add a "new drawing" while inside of Google Docs.