Creating an undulating shape on top of a ring

There are several ways to do that.

Curviloft extension could do that pretty easy.

It seems you’re working with the default value of 24 segments for a circle, you can change this number typing the number you wish followed by “s” character and then hit return. For the example of the video I typed 96s

@jimhami42 tells you here more about how to figure out the right number.

Some tips:

When you use Push/Pull tool you can use the Ctrl as key modifier to create new shape instead of pushing or pulling the existed one.

You may find useful in certain moments:
* X-Ray view
* Parallel Projection for certain

To find the center of a circle you have to hover the edge for a sec, then the center will appear as selectable point.

To find the center of a rectangle you have to hover the middle points of each side.

You can intersect with the selection or the model and then erase what you don’t need.

In the case of the video I drew a highest arc and it’s so long that I had to erase the one from the other side. You could use this alternative way:

Convert the cylinder into a component to avoid sticking, then move the arc extruded to the position you want, you can even make it shorter with push/pull tool, and then before using the intersection option you have to explode the component, you can right click and choose explode for that.

X-Ray view will help you to delete internal faces and edges and the eraser tool used with the Ctrl key modifier will help you to smooth the edges.

Regards.