firstly you have to literally MASTER 90 degree hold to the point where you do it without any trouble secondly you have to master handstand push ups so you can randomly do one at whatever time then practice a lot of 90 degree presses to handstand and do these steps while trying the skill: do negative handstand push up, while you are on the lower part of hspu put yourself in a 90 degree position, then put yourself in the lower position of hspu, and do literally the same move as you would in regular handstand press I'm always doing it that way, and it looks fine:

Start off by doing 10 clean pull ups and you will be fine