after you do your light or medium scissor kick, push [up-back + kick]. this way you retain your back charge for the PC while performing your headstomp at the same time.
on akuma’s wakeup he’ll either block the HS, which means you do whatever follow up you want to do after an HS and you can’t possibly do an expc, or he teleports away, which is when you push the direction facing toward akuma and two punch buttons so you execute an ex-pc.
it’s not really on option select so much as it is abusing the lenient input time and recognizing that akuma tried to escape you
fairly certain an akuma mashing m.dp on wakeup will beat you outright, and mashing l.dp will trade you