How do I sign and return an email someone has sent me please

Is this something that you physically have to sign? If so, there are ways, but the most common way is to print it out, sign it, then scan it. Then it can be sent as an email attachment. The best way to do that is with a computer and a printer/scanner.

Or is it through some automatic document service like DocuSign?

You may have to save the document to notes first then add it back to the email before you return it.

Sign a document

1. Open Notes, then tap the document in the note.
2. Tap the Share button , then tap Markup .
3. Tap the Add button , then tap Signature  and add a saved signature or create a new signature. Then you can adjust the size of the signature box and place it wherever you want on the document.
4. Tap Done.

To manually sign your document, follow steps 1-2, then select a tool to use and sign the document with your finger or an Apple Pencil with a compatible iPad.

Is there an attachment in the email?

What issue are you having. I am not done helping.