Making a spiral staircase in a block-based building game seems difficult, but it can be done!
The first challenge is the circle: spiral staircases need a circular trace. Fortunately, this page has circle diagrams for Minecraft!
Here's what to do:
Step 1: Make a circle of any radius
Step 2: Make a bigger circle around the first circle (they should share the same center!)
Step 3: Fill in the area between the two circles and elevate the blocks
Step 4: Use your judgement! Sometimes this method results in kinks in your staircase. If you feel adding an extra block will improve your staircase, go for it. Add to many blocks, however, and your staircase will become choppy.
Example from Creative mode:
Examples from my Survival world:
Step 5: ?????
Step 6: PROFIT!!
Enjoy! Please comment and critique. Pictures will come as soon as I figure out how to upload them (I'm relatively new to the forums). Pictures are here!
EDIT: I will be making a more in-depth tutorial when I have time. Expect one soon!
Slabs are probably better for this purpose, they handle diagonals more smoothly.
This guide is accurate enough, but not that informative; steps 3-4 are pretty much saying "Make the staircase look right by making it look right". Some tips on making diagonal steps decent looking and easily walkable would be useful.
For example, Leostereo's mansion tutorial where he explains how to make corners for stair-based roofs (slabs for external corners, full blocks for internal ones).