Open your Google Doc
Place the cursor where you want the signature
Click Insert
Select Drawing
Click New
Click the Line tool
Choose Scribble
Draw your signature with the mouse or trackpad
Click Save and Close
Resize or move the signature if needed
Alternatively, insert a signature image by clicking Insert
Select Image
Upload from computer or choose from Drive, Photos, or URL
Position the image where you want it
Alternatively, use an e-signature add-on from the Google Workspace Marketplace