I have been playing yoyo for a month or so now but i still can throw a straight and strong sleeper
In the video he say not to move your palm downwards but i keep doing that… what should i do?
I find that throwing the yoyo outward rather then downward, it is a little straighter.
well when you throw, look at the yoyo to see if it’s tilting. bind and return. then based on the tilt, turn your throw hand appropriatlety(sorry for my spelling lol)
it’s about practicse. your throw will improve eventually
Are you saying that you throw with your palm facing down? If so, you don’t want to do that. You want to have your palm facing up.
As for the straight part: If your throw is tilting one way, do like AeroSlaYa said and adjust where you aim your throw to compensate for the tilt. Eventually you’ll be able to throw down regularly and not have the tilt. 
and with the palm thing i do that all the time and nothing’s really different. throw with palm straight up but then as soon as it comes down my palm also faces down and yet the throw is straight remember practice and you’ll get it
Try to flick your wrist really hard. You should use your wrist more when you throw a sleeper than your arm. By the way this goes in the trick section.