What are the steps in building a 5ft diameter flower garden

Your question was cut off, can you repost please? 5' diameter what?

you question is not complete

stairs, or similar to retaining wall?

I would take a hose or florescent lawn marking paint and draw your circle where you want it. Remove any sod, turn the soil and amend it with some compost and peat to make it rich. If you want to keep it weed and grass free, put down landscape fabric, cut an x with a box cutter where you want to insert a plant.

I would add a border around the bed, could be rocks, bricks, bender board etc.

You might want to add some garden decorative piece in the middle like a gazing ball, bubbling fountain, bird bath, garden ghomes, or flowering shrub. You can make your plantings in many designs, like concentric rings, spokes like a wheel, plants from high in the center to low at the outer circle. https://www.google.com/search?q=pictures+of+circular+flower+beds&client=opera&hs=n69&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiopMDIyNrVAhUC9WMKHWffDtYQ7AkIQg&biw=1366&bih=629