You don't actually confirm the Yeti. You just need to get the photo of the footprint, which is not enough proof.
When you have proven the existence of the other three cryptids (Nessie, Chupacabra and the Jersey Devil), Mews will send you in pursuit of the similar creature, the Bigfoot, in the Pacific Northwest.
Bigfoot is obviously confirmed at the end of the trip there.
When you have reviewed all 5 videos in the museum, Mews will arrive, show you the lab, and ask you to hunt down the cryptids.
After you watch all 5 videos in the Cryptids Museum, Mews will return and give you some more instructions. Then you can fly off to locations in the helicopter.
It means that there is scientific evidence that the cryptid exists, which is what you are trying to do for the 4 unproven cryptids. When all are confirmed except the Yeti (and you have the footprint photo), Mews will send you in pursuit of Bigfoot, who is soon confirmed.
Watch all the myth videos and let the guy brief you on the mission.
The heliport at Mews' house is out the door, then to your right. You cannot use the helicopter until you have reviewed all of the videos on cryptids and spoken to Mews.
He should appear in the museum or the lab (to the left) after you have watched all of the videos in the museum.
Go to the museum (first floor right) and watch all 5 of the cryptids videos (at least part of each one).
He appears only after you have watched all of the videos in the room (you don't have to watch them all the way through).
After Mews leaves the bedroom, drink the water and go downstairs to the room on the right, which is the cryptids museum. Look at all 5 videos there (at least part of each one) and then Mews will appear to offer you the chance to track down four of the cryptids.
When you come downstairs in the Mews mansion, go to the museum (first floor right) and view all 5 videos about cryptids. Then Mews will appear to give you instructions.
In the Mews mansion, go to the museum at the right side of the first floor. Once you have looked at all 5 videos, Mews will arrive to give you some instructions and then you can take off in his helicopter.
You awake in the Mews mansion. After he leaves, drink the water, go downstairs, and enter the museum (first floor right). Review all 5 of the cryptids videos and then Mews will join you.