You tame them by droping ferti eggs from your inv on the ground. You need to do so when the snake is not aggroed, so sit on a rex or smthing it wont attack. Be sure to drop them from your inventory, not from dino ones, that will make it bugged, it wont eat. To a lvl 20 Rex egg gives 29% on 2x event, bronto 26.7% and giga and wyvern the same, looks like all the big eggs give approxymately the same amount.. so dont waste the best you have ? If they get aggroed on you just run away and return, taming wont drop. Happy taming!