Choosing the picture
Take a harbour, a market stall, a row of houses. What they have in common is hard edges and separated colour, which is what makes a guide that thirty children can all follow.
Avoid the things that look appealing and reduce badly. A sunset is a gradient, so it arrives as accidental stripes. A woodland scene is fine repeating texture, so it arrives as confetti. A close portrait puts almost all the detail in one small area and leaves the rest of the page empty.
Use an image you are allowed to use, and if it is a photograph of the children themselves, that is exactly the case where it matters that the picture is analysed on the machine in front of you rather than sent anywhere.
The settings, and why
Shape Colouring with hexagons rather than Contour Painting. This is the decision that matters most and it is counterintuitive, because contour tracing produces the better looking guide. A repeating tiling gives every child cells of a similar size, so everyone has comparable work. A contour guide gives one child an enormous sky and another child forty small shapes, and the second child will not finish.
Eight colours. Not because eight is artistically right but because eight is roughly the number of paint pots that fit on a shared table without anyone reaching across somebody else's work.
Detail low. Large regions survive a photocopier, tolerate an unsteady hand and let a child see progress. The readability warning is the thing to watch here, and if it fires a lot the answer is to lower detail further rather than to print anyway.
Line weight one step above the default, for the same photocopier.
What to print, and how many times
Thirty outline sheets at A4, at 100 per cent. Not scaled to fit, which quietly undoes the physical sizing the guide was checked against and shrinks every region by a few per cent.
One colour answer sheet, in colour, on the wall. This is where most of the printing budget for an activity like this usually goes, and it does not need to. Thirty colour prints cost real money and one on the wall works better, because children look up at it together and talk about it.
One spare set of five sheets. Something will be spilled on.
Build the guide at the size you are going to print. A guide made for A4 and then enlarged to A3 for a display arrives with line weights designed for the smaller sheet and looks thin and washed out.
How the hour goes
Ten minutes at the start looking at the answer sheet together and naming what they can see. This is the part that gets cut when time is short and it is the part that makes the difference, because a child who has understood that the picture is made of areas of one colour will paint it differently from a child who is filling in shapes.
Then paint the largest areas first, everyone together, one colour at a time. Calling the colours rather than letting thirty children choose independently keeps the pots moving and stops six people waiting for the blue.
The last fifteen minutes are for the small shapes and for the fact that a third of the room will not finish. Say at the start that finishing is not the point, or you will spend the end of the lesson managing disappointment.
Adapting it without preparing three activities
The same photograph at three detail settings gives three sheets that look like they belong to the same lesson. That is the quiet advantage over buying worksheets: differentiation without producing visibly different work, so nobody is holding the obviously easy one.
For a group painting one large picture together, a low colour count block in on poster tiles gives you a wall sized version. For a child who finishes early, the same picture at higher detail is a second sheet rather than a filler task.