To redeem your Totem Stickers, you will need to travel to Heahea Beach on Akala Island. That’s where Samson Oak loves to chill. If you pay attention you might see his name on the bulletin board for Surf!
Whenever you get enough Totem Stickers, Samson will also call you to remind you, but you are free to redeem your stickers any time. Note that the Totem Pokemon you get are mostly version exclusive. If you want them all you’ll have to trade or own both versions.