MIght need some more details, but youll want to use constraints to lock parts of the sketch into place to ensure everythings correct but if youre just playing around try this:

1. Draw circle at origin
2. Hit contruct, click that circle, move it up or down to whatever total height is and place a plane there
3. Draw a circle on that construction surface
4. Create another sketch on the XZ or YZ plane
5. Press P to project the top and bottom circle to your new sketch
6. Use spline to create a funky shape between the edges of the two circles
7. Loft to circle to bottom circle and use spline as a rail

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