j.HK, 4HP > 214P~LK > 28MK
j.HK, 5MP, 2MP > [2]8MK
The first combo may be easier to start with as you don't have to start charging during a standing move. That being said, 5MP 2MP is a very important link to learn. Both combos end with MK Spinning Bird Kick, which provides Chun with good okizeme and corner carry.
How do you actually do that second combo? Spinning Bird Kick will not have time to charge if you only hold down when you press 2MP. What you actually need to do is to is hold [2] right after 5MP is pressed. Doing that, you will discover it's actually pretty lenient.