Pick a task on the left

You get a start sheet and a picture of the target. Make yours match: values, formulas, number formats, bold, borders, colors, column widths, frozen panes, groups and sheets are all compared. A mouse click on the grid flashes red and adds two seconds instead of failing you, so you learn to stay on the keys.

The sheet checks itself after every key, so there is nothing to submit. Ctrl+Shift+Enter shows what still differs.