Usually when you load procedures, like arrivals (STAR) or approaches the altitudes are populated automatically. Are you seeing this?
Maybe focus on learning one set of avionics at a time. I found the CJ4 easy to learn but if you rather have something slower start with the G1000.
Basics:
You can also manually enter altitude constraints. There are three types:
- at: VNAV will target this exact altitude at the waypoint
- below: VNAV will calculate a path that will keep you at or below this altitude at the waypoint
- above: you get the idea.
What VNAV does is calculate a path from your current altitude down to the last restriction in your flight plan while meeting all restrictions along the way. Based on your current altitude it will calculate when you should start the descent, marked as top of descent (TOD).
Assuming your autopilot supports this, you can engage VNAV. This usually arms VNAV PATH mode. This will automatically start your descent and adjust your rate to meet all restrictions. However, it will only descend if you lower your altitude selector. Note that VNAV will never go lower than what you manually select, even it would violate a restriction. It’s up to you (and ATC) to make sure you’re cleared to the correct altitudes.
So, once you got your restrictions loaded, look at the last restriction and set your altitude to the same value. Then engage VNAV. Once reaching TOD, you should see the plane starting to descend.
As I said, you should pick a plane and a flight plan and we can walk through it and I can help you to get it to work for a specific case. There’s lots more, especially when it comes to RNAV approaches.