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
Click Save and Close
Resize or move the signature if needed
Or insert a signature image by clicking Insert, then Image, then uploading your signature file
Or use Insert, then Drawing, then New to add a scanned signature image
Or use a third-party e-signature add-on from the Extensions menu