Help with sitting properly in the saddle

I am looking for some advice on sitting in the saddle properly. I seem to struggle at a lope especially when turning on a barrel or pole etc. I am not sure what I am doing wrong but a lot of times I feel like I am light in one of the stirrups. I am not sure I have my seat figured out.
I have been told inside seat outside leg...not sure what that really means " inside seat" Does anyone have an ideas that may help me ride better?

How I usually lope is heels down and my hips turned inward(which way the lead is going position them left on left circle) try to find her/his stride pattern is the hardest thing...I invasion it like my hips make a circle... Hard to explain but gotta try it:)

It would be so much easier if you could post a video. Worth a thousand words.

And make sure you are riding horse, not saddle.