Learn English – Which to be used; “I only” or “I theself”? Or something else

grammar

What should I say when I want to convey that the job was done by me when asked by someone?
(In the context of no one but me)

I only did this or I myself did this?

It depends on the context.

If replying to a simple question

Who did this?

I did it.

If you are wanting to emphasize that it was you and not someone else:

Did you pay someone to write your thesis?

No, sir, I did it myself.

If you want to emphasize that you are the only one:

Did John help with the yard work?

No, he played games all day and I did it by myself.