Jump forward and hold back. Do that as one motion. Don’t wait until you see your character jumping. If you wait for that, then it’ll take you too long to start charging. Just do it as one motion and have faith that your character will jump. If you fuck up the jump motion (hey it happens to everyone now and then), you’ll have plenty of time to react and just block. In fact, since you’re holding back anyway you’re probably not going to get hit. Anyway the point is that you have to do “UF, B” as one quick motion.
Then do j.HP, s.HP and make sure you get the timing right. Try to do j.HP semi-deep so that you don’t have to be ultra-precise with s.HP timing. Once the s.HP connects, WAIT a little bit before doing the Sonic Boom command. If you rush it, you might kara-cancel the s.HP into Sonic Boom and of course that’s not going to combo off the j.HP if the s.HP never connects. Thing is, you should wait about 10-15 frames after pressing s.HP before you do the Sonic Boom command. You have a lot of time and unless you’re trying to do some fancy combo that requires multiple charge moves, there’s no reason to rush things. So wait for a while and then do the Sonic Boom command. Cuz if you’re having trouble charging in time, then that extra 10-15 frames will make it real easy. If you STILL have trouble charging in time, then you’re definately doing something wrong. But you can make it a little easier on yourself by superjumping instead of normal jumping, cuz superjump has a little bit longer airtime. If the opponent is dizzy in the corner then it doesn’t matter whether your normal jump or superjump, so you may as well superjump and make your life easier. Again, do “D, UF, B” (or “DB, UF, DB”) as one quick motion.
Also, make sure you do the Sonic Boom input right. Some people get too anxious to start recharging that they go from holding B to F, then hold B again too fast. Sometimes they never properly hit the F input. This happens even more often when trying to combo into supers. If you’re having trouble with charge combos, try to focus on getting the combo right before you worry about re-establishing charge.
Anyway this is all super basic shit but i don’t think people bother to write it out nearly often enough. After all, way more tournament matches are lost because of people fucking up basic shit than fucking up advanced shit.