AutoCalib — camera calibration without any designed target

Choose the sharpest image where the flat calibration target covers as much of the frame as possible — it doesn't need to reach the very edges, since you'll mark the exact region to use in the next step. This image must also be shot roughly straight-on, with the camera close to perpendicular to the target — it doesn't need to be exact, but a clearly tilted reference image will cause calibration to fail or produce inaccurate results.

Shoot the reference image with the camera’s optical axis roughly perpendicular to the target plane — it doesn't need to be exact, just close, no need to fuss over it with a level or tripod. This is still a real requirement, not just a suggestion: calibration will fail or become inaccurate if the reference image is noticeably tilted, since it is the geometric anchor for the entire calibration. Frame it so the flat target fills as much of the image as possible — it doesn't need to reach the very edges, since you'll mark the exact region to use in a later step, but the more of the frame it covers, the better. Pick a target that's physically large enough to fill the frame at a comfortable distance, rather than getting there by shooting a close-up: the reference shot is typically also the one you focus on, and since focus should stay fixed for the whole set, locking it at a close distance can leave your other images — taken from farther away — outside the depth of field and blurry.

Draw the polygon so it lies entirely inside the flat, textured region of the calibration target — it doesn't need to trace the boundary tightly or precisely, but it must not spill past the edge of the flat surface onto anything else. What matters most is that the marked area covers a large fraction of the reference image, rather than a small patch in the middle.