This is called an "otg" combo, depending on whether a hit is performed "on" or "off" dicates it's meaning. In this case you perform the knee lift combo - HK LP, as soon as your opponent slams onto the ground. That's the best way to time it. To enable some better juggling properties afterward, you can put a slight pause between the HK and LP, this way, the uppercut hits them when they are slightly higher, causing them to go higher.
-_-'' Matt Matt matt... Matt, I was teaching you combos the other day! you could of asked about the OTGs, just do it a little late, and you'll get the timing, but never run in, unless your good at it, for the first time, do it close, and walk in then do the Pop-Up.