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