built a trap near its nest, in scorched earth just prepare some near the world scar, you'd usually need some extermination dinos to clear out the surroundings so they don't mess up your plan when building traps or trapping and knocking them out, then just lure one out and make sure to fly slightly or beyond their breath reach and you have yourself 5 wyvern milk, make sure to have preserving salts ready or just immediately place them in a tamed wyvern's inventory, unfortunately they don't get un-trapped after they wake up so you either need to unload the area to release them or just kill them so new ones spawn, though if you're playing on a server especially crowded ones it doesn't unload and escape