Help

[Presentjackson]

Do i have to wait for my opponent to put something on the stack to cast a creature with flash? 

[Jesse Shredister]

No, it's basically an instant spell now.

[Stolen711]

You have to have priority. If it's your opponents turn and he doesn't want to cast a spell he has to pass priority to you before the turn can move onto the next phase. At that point you can cast your flash creature. 

[Presentjackson]

Thanks. I should have clarified. Thats the situation i was refereeing to. It's my opponents turn. They untap/draw and have priority until they want to cast a spell or try to attack. 

[Stolen711]

During their first main phase they have priority to cast spells and they have to pass it to you before moving onto the beginning of combat step. So they cannot attack you until you have the option of casting your flash spell during their main phase or even during their beginning of combat phase. You could even wait until the declare attackers phase and after they have declared attackers flash in your creature as a blocker after they have passed priority. That  has to be done no later then to declare attackers phase because if you flash it in during the declare blockers phase it's too late to declare that creature as a blocker.

[Kaleo42]

Youre first opportunity is when they pass priority during the upkeep. Next is the draw step after they draw ({quicken} + {thoughtsieze} great here). Then main phase, attack step after attackers are declared, blocker step after declaring blockers, response to moving to damage (both times if first strike is involved), end of combat, second main, end step. All when you are given priority (which they have to do to change phases).

[Stolen711]

+ beginning of combat step before declare attackers 😀

[Presentjackson]

Awesome! Thank you. Sometimes its tricky knowing when to play your cards. I had a {skyline predator} and i casted it after my opponents draw phase. Not only was it stupid and hasty, i thought it might have been illegal. But it seems you past priority after everything you do