How a map draws your build
A map at its closest zoom shows a 128 by 128 block area, one pixel per block, seen from straight above. Each block has one map colour in the game's data, and the map draws that colour in one of three shades. When a block is level with the block just north of it the map uses the plain shade; when it is higher, a lighter one; when it is lower, a darker one. This page knows every block's map colour and does the same sums, so the preview is what the map will show. Checked against Java Edition 26.2; the map colours have not changed since 1.17.
Flat or staircase
Flat builds every block at one height, so each colour comes in only its plain shade: 60 colours with water set aside, fewer once you leave some blocks out. It is quick to build on flat ground. Staircase uses all three shades, nearly three times as many colours, by raising and lowering blocks. Going south along a column, a lighter pixel steps up one block, a darker one steps down (any amount, so a run of darker pixels can drop back to the ground), and a plain one stays level. The page works out the lowest heights that give every pixel its shade and shows the tallest column. It is often 10 to 40 blocks for a photo, but a long run of lighter pixels in one column can climb as high as 129 blocks for a single map, and more across several maps down.
The north edge row
The first row of the picture is shaded against the row of blocks just north of it, outside the map. Lay one row of any block there, level with the first row of a flat build, or at the heights the plan gives for a staircase build. Without it the whole top row comes out lighter or darker than planned. It is counted separately under the materials, since it can be any block.
Where the map area is
Maps line up with a fixed grid in the world. In Java Edition a map made at full zoom covers 128 blocks each way, from x = 128n − 64 to 128n + 63 and the same for z, so one map's area runs from −64 to 63. Stand inside the area you want, make the map, and build within it with the top-left of the picture at the north-west corner. For several maps, build the areas side by side; each map shows its own 128 by 128 part of the picture.
Dithering and the blocks
With Dithering on, the difference between each pixel and the colour chosen for it is passed on to its neighbours (Floyd–Steinberg), so smooth gradients come out as fine mixes instead of bands. Turn it off for flat logos and pixel art. Tick the block sets to draw from; a colour several blocks share uses the first one ticked, and the menu in the materials list swaps it for another. Leave out hard-to-get blocks drops diamond and emerald blocks, prismarine, End blocks and silk-touch-only blocks. Leave out falling blocks drops sand, gravel and concrete powder, which fall unless something holds them up, as they would on a staircase. Carpets give the same colour as their wool for less; on a staircase each needs a block under it.
Picture, PNG and limits
Crop to fill trims the picture's edges to the shape of the maps; Fit it all in keeps the whole picture and fills the rest with white. Up to 4 by 4 maps, 512 by 512 blocks, worked out in the background so the page stays smooth. Download PNG saves the preview, each block as a sharp square, to compare against the map in game. The picture is read in this page and never uploaded.
Questions
Does this work in Bedrock Edition?
Not exactly. The colours and the height rule here are Java Edition's. Bedrock Edition draws some blocks in different map colours and shades its maps its own way, so a plan from here can come out patchy on a Bedrock map.
Why is water not used?
Water is shaded by how deep it is, not by the block north of it, and needs a container of blocks around it. This planner sticks to solid blocks, which give the same result every time.
How many blocks does one map take?
A full map is 128 by 128, which is 16,384 blocks: 256 stacks, or a little under 9.5 shulker boxes of 27 stacks each. A staircase build needs the same number of picture blocks, plus whatever you stand them on while building.
My map shows the top row the wrong shade. Why?
That row is shaded against the blocks north of the map area. Add the north edge row described above, at the same height as the first row for a flat build.
Can I export a schematic or a structure file?
Not yet. Build from the preview, the hover readout and the map-by-map lists, or the PNG on a second screen.
What about pixel art you look at directly?
Art built standing up or on the ground for players to see uses blocks' plain colours with no shading. The Pixel Art Generator plans that, from a picture or from words.