FTM questions for moms giving solids!

Hi ladies! I'm a FTM and I have some questions about starting baby food.. my LO is 19 weeks/20 weeks on Friday.. and I want to start buying some gerber first foods for him..I know some moms want to wait till 6 months. That's fine. My pediatrician gave me the go ahead at 4 months a long time ago so I want to try it.. my questions are for those of you who are already feeding foods to your LO. 
1. How much/how little a day?
2. What did you start with? Fruits? Veggies? Rice cereal? (If with rice cereal how much and how did you mix it? And for how many days? Or every day?
3. I know they say to try out each food for 3-7 days to rule out allergies so do I feed him the same fruit or veggie every day for 3-7 days? Once a day? Or do I feed it to him once and then wait those days? I'm confused lol
4. How much fruit/veggies do you feed them? The whole container? Or just a couple of tsp? What do you do with the rest? Store it in the fridge? How long can I keep it in there before it expires? ( I'm talking about the small containers of the gerber food, the first foods for supported sitters)
5. Does feeding them solids constipate them? My LO has a history of getting constipated a lot when he was younger..
Those are all the questions for now.. I'd really appreciate any and all advice! The more detail you can give to help me out would be great! Thanks moms! 😊