Open your Google Doc
Click where you want the signature
Go to Insert
Select Drawing
Click New
Click the Line tool drop-down
Choose Scribble
Draw your signature with your mouse or trackpad
Click Save and Close
Resize or move the signature if needed
Or go to Insert
Select Drawing
Choose From Drive if you saved a signature image
Upload a scanned signature image and insert it into the document
Or go to Insert
Select Image
Upload from computer
Insert a signature image file into the document