Mismo is an emphasis to whatever pronoun which denotes a certainty of an action by or fact about that pronoun.
Q: Ito ba ang kinain mo? / Is this the one you ate?
A:
Iyan *mismo* ang nakita ko.
That *exactly* (or, itself) what I saw.
Ako *mismo* ang gagawa.
I will do it *myself*
Sarili is used to refer to oneself. Usually to persons, but sometimes to things as well:
Kilala ko ang *sarili* ko.
I know *myself*.
Lumipad nang walang gumagalawa *sa sarili* niya.
It flew unprovoked *by itself*
Sometimes, mismo and sarili may be put together for greater emphasis of certainty:
Sarili ko mismo ang nakita ko sa salamin
It was me, myself, who I saw in the mirror.
The words "ko" "akin" "niya" and similar qualifiers will always be with "sarili" to qualify it.
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