Step 1: Go to the Menu and click "Island Scan" It should be marked a QR code.
Step 2: Scan 10 QR codes. To find QR codes look up "Island Scan QR Codes"
Step 3: It should tell you there's a Pokemon in a certain area of the island, go there.
Step 4: Catch it. If you want Serperior go use Island Scan on Thursday at Poni Island.