How do I sign a PDF in Okular?
To my knowledge, currently none of Okular's backends support electronic signatures, although that feature has been requested a number of times.
As an alternative, a PDF (or any other file) may be signed using a detached signature with GPG or any of its numerous frontends (such as Kleopatra or Kgpg in KDE).
I have a .png of my signature, and I basically want to insert it into the document on the dotted line
Do not do this.
Anyone having access to your PNG or any document where it is used, such as the PDF you are intending to embed this on, will have a perfect, infinitely reproducible copy of your autograph signature. It offers no security at all and in fact is detrimental to it: upon seeing your scribble on a document, recipients may be tempted to assume that it is authentic and not bother to check for an actual (and legally valid) electronic signature. Acrobat is, or at least used to be, a major offender in this regard—I have seen documents trivially "forged" because of this ill-conceived feature.