I'll try to help you here.
Let's start with the orbit in your latest screenshot. The grey circle is the Earth, the green ellipse is your orbit. It goes too high. So we'll going to make it lower.
Press the mode button until the many parameters appear. Don't be scared of them, you won't need too many. Check out the PeT. That means time to periapsis, how many seconds are left 'till you reach the lowest point in your orbit.
When you get quite close (approximately 200-300 seconds) turn retrograde. The shortcut key is ], or you can use the button on the panel.
Once the ship orients itself to the target, wait until you get close enough to the periapsis so you can make the burn. The more you want to change your orbit, the longer the burn. The longer the burn, the farther away from the periapsis you'll start. This will be quite a long burn, let's start the burn at 20 seconds from periapsis.
When PeT reaches around 20, ignite your engines. Keep them on. You'll see the long, green ellipse becoming more and more like a circle. Keep the engines on until it's quite circular. (By the way, there is one parameter on the Orbit MFD, the Ecc parameter, which tells you how circular the orbit is. The smaller the number, the rounder the orbit. Try to bring it as close to zero as possible.)
Now, if you want to adjust the apoapsis altitude (highest point of your orbit) then watch the ApA parameter. Say you want to bring your apoapsis to 350 km. So you'll put tha ApA at 350 k. It's not that hard, you know
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If you don't do it in one burn, don't worry. You'll always come back from the other side!
Hope this helps! If you've got any more questions, don't hesitate to ask!
