How do you get the yeti skull in poptropica?

You don't. Other than the lantern and the footprint photo, there is nothing to do at the Himalayas.

Later, when you have proved the existence of the other three cryptids (Nessie, Chupacabra and the Jersey Devil), Mews will send you to the Pacific Northwest to find Bigfoot, which is much the same creature as the Yeti.