ok here's front lever:
like you said, you can do it on the high bar or rings.
first start from an inverted hang (upside-down handstand, with arms to your side), lower your legs/body to horizontal while in the hollow position, toes pointed, keep your arms straight - pull down towards the bar or rings and inwards in front of you, shifting your upper body backwards to make your center of gravity, until you are horizontal to the floor (abs facing ceiling, back facing floor)
-easier on rings than bars b/c on rings there's no bar you have to avoid hitting, and your hands can rotate on the rings
-->need pretty good shoulder flexibility / strength in chest and shoulders / good range in motion
back lever: ...pretty much the opposite:
on rings or high bar,
start with inverted hang with an underhand grip (if you're doing it on a bar, the bar has to be behind you), pivot your shoulders until you are parallel to the floor (abs facing down, arms at about 45 degrees angle), rings should be above hips
Planche: handstand where your body parallel to the floor, or, push-up position where your feet is off the floor and your body is parallel to the floor - your upper body will be in front of your shoulders to keep balance