A thin circle is mostly straight runs and a few places where the outline turns. On a straight run the two outline neighbours sit opposite each other, and this page marks that cell S, for a slab. Where the outline turns, or a cell has a neighbour that is not part of a straight pair, the cell is a stair. The letter is the first letter of the direction that stair faces, and the facing is away from the centre: E or W when the block is farther from the centre on the horizontal axis, N or S when it is farther on the vertical axis. Positive Y, the lower half of the picture, is S. Negative Y, the upper half, is N. An isolated cell, with no outline neighbour, is B for a full block. None of these letters is a Minecraft block id. They are a placement hint so you can decide slab, stair, or block while you look at the grid.
The page opens on radius 8 and thickness 1, which is 44 blocks, the same thin circle as the plain circle example. Numbers are off, because a number and a letter cannot share the cell. Load example puts that 44-block ring back if you have changed the radius. Raising thickness turns the letters off. A two-block wall is an outer ring plus an inner ring, and calling the inner ring a slab would be a guess this tool will not make. Stay at thickness 1 while you are reading S and the facing letters. If you only wanted the silhouette, the circle page draws it without letters.
Build the four axis ends first so the radius is obviously the one you typed. Then walk a straight run and place slabs where the grid says S, and turn the stair so it faces away from the centre block when the grid says E, W, N, or S. The row count under the grid is still the number of blocks on that east-west line. The letters do not change the count. If a row on the left has a different count from the row on the right, you have drifted off the centre. The outline is symmetric.
Slabs and stairs in game also have a top half and a bottom half, and a stair has a shape that depends on the neighbours the game itself sees. This grid does not know whether you are building the ring as a floor, a window sill, or a battlement. S means "this cell is a straight run, a slab is the usual choice." It does not mean the game will accept a bottom slab in a place where you needed a top one. Corners where three blocks meet can still look wrong with a single stair. Treat the letter as the first try, then nudge the corner in game if the turn eats a block.
Thickness, when you do want a wall you can walk, is on the thickness page. A numbered outline, for a screenshot you will talk through, is on the numbers page. Both start from the same midpoint circle. The stair letters are the extra reading, and only while the wall is one block.