Planning: open your favourite Paint-type package. (For the purposes of this walk-through, I shall actually use Paint. XP version, if that matters.)
Open paint. Change canvas size to 100x100 (or required size) through Ctrl-E and inserting the given width and height. Zoom in (Ctrl-PageDown, or View, Zoom, Custom and choose the zoom level you want to fit within the screen; you could also enable gridlines, but probably not necessary). Make sure your "line drawing" is at the thinnest level (you can bulk the walls out manually, and manually close the 'open diagonals' by augmenting with your choice of "inner corner" or "outer corner" walls at those points, according to taste), then switch to the Oval tool, to draw just the border. Take your mouse and click on the top left corner pixel (0,0) and drag out until your circle's right and lower extremities touch the right and lower edges of the canvas. Release. Result: One 'circle', which you can generally translate into DF.
Zoom in more if you need to examine the pixels better, on larger designs. You can reduce the canvas size by half (Image Attributes or Ctrl-A, not Image Stretch/Skew or Ctrl-W!) to get just one quarter of the circle, if you want. If you want to augment it in a regular way (e.g. with internal room layous), concentrate on just one eighth of the circle's segment (say N to NE) and then copy the image, paste and then use the with three successive 90 degree rotations and one reflection (in either axis) to propagate the same pattern around the other segments. You could use various colours for the improvements so they're easy to wipe[1] some bits (e.g. internal walls) without wiping others (e.g. external walls and already placed stairwells, or doors from all doorways, that sort of thing).
There's at least one tool (probably several) specifically for designing DF plans, out there. Not used it myself, so not sure if it helps with constructions, or just mining out internal and underground layouts. I've also not used the Paint solution, but it should work for you.
As for advice on the building process, which you don't ask for but I'll give anyway, you might also want to set up specific stockpiles for the wall-material of choice (if you're picky) next to (but not on) the line of the walls, to allow the construction masons to quickly zip back and forth between the stockpile and their building sites. Of course, this means you should only designate walls to be built if you can designate the material you want from an item within short range, otherwise you're maybe sending them down to the underground quarries (or other material source) when you could be waiting a moment for a hauler to get you the material within quick range. Or send them there anyway for a few stones/blocks/bars/logs, on the basis that while they're hauling and building one item, a number of other s/b/b/ls are being hauled into range by the peons so that the next few walls to be built are quicker. Look at it as a Time And Motion study, I suppose. If you're dealing with a curve, as you will be, I might also be tempted to designate (and make sure are built) all the "external corners" of the circle first. In the 100x100 example I just drew myself there's 280 wall segments (not including "diagonal-fillers"). But only 80 of these (if you account for the almost '45 degree' segments centres as being the same as orthogonal wall centres) are 'corner' blocks, which you can place and then easily fill the remaining gaps in an intuitive way, without needing to further refer to the basic circular plan. I also tend to do the same with wall-junctions (in my plans, mentally held, for various large-scale complexes needing internal partitioning), especially as then you can designate all the non-junction internal walls without worrying about sealing up the central pillar in a crossing point, unable to be constructed because the four outliers already have been. And if you're worried about defence, the biggest priority (first the corners, as discussed, then the walls between) would be the outer skin and any drawbridge points or other sealing methods (doors, floodgates, etc) that you are wanting, then get to the internal cross-junctions/corners/door-jambs and then the remaining walls, floors, furniture, etc.
For the material, you can also be OCD about it being all one kind, or take what you can get. Or you can make the bits that aren't of the right material out of obviously the wrong material so that later on you can selectively (as long as you haven't blocked them in, and they aren't the sole support for anything on a higher level, for example) deconstruct and reconstruct accordingly. No having to differentiate feather-tree wood from marble, or even raw marble and marble blocks (under default tile-set, they'd all appear bright white, although they'd look different under an added tileset), or anything along those lines, if you really want it all to be the same.
Really, it's all up to you, anyway.
And ninjaed by Doomshifter, somewhat. Although, quickly looking, some of those (e.g. 19) have got some "filled diagonals" alongside some non-filled ones. Others (e.g. 21) are completely unfilled. Slightly inconsistent, but you probably already know that.
[1] With foreground in that colour, background in the regular field colour, Flood-Fill the base area with the colour you want to remove with left-click then flood-fill with right click to revert the field and bring the pixels originally of the foreground colour back to white/whatever... Repeat a few times if you need to 'bleed' into some enclosed areas to reach internally isolated/diagonal-only connections... if you've got other pixels of another colour in the way, move the mouse to the other side of the barrier... Easy enough. And there are easier ways still, for other situations, but this one's generally good enough for most circumstances.