I start my car in first it doesn't matter what gear you start it in. Going to neutral doesn't save gas, not pressing the gas is the same as being in neutral. You shouldn't need to hit the gas to get the car rolling just slowly let off the clutch and you'll feel it start to go forward. I'm usually rolling before I even hit the gas. Whe you downshift and you're going fast you have to match the rpms of what the next year down will be. In my escort say I'm going 50 in 5th gear if I wanted to downshift to 4th and keep going 50 I would have to rev it up to a little over 3000 rpms before I let the clutch out in 4th. You get better with practice and the jerkiness goes away. I can take off in 3rd gear in my car, I didn't even think it was possible. Not in an escort anyway. I've done it twice both on accident because I forgot I stopped in 3rd.
Its a matter of time spent. No matter how much time you think about it and analyse the situation until you get more seat time rowing the gears you won't gain the knowledge you are looking for. Driving a manual transmission in every kind of circumstance will grow you confidence - time to get a stick shift and start rowing.