Use the arrow keys or click to navigate, and use the number keys to fill in values. Singleton entries are treated as pen (final answers) and multiple entries as pencil marks. Use space to delete, 'u' to undo, and 'r' to redo.
Fill in the squares with numbers (1, 2, 3, et cetera) such that no number appears twice in the same row or column. Look at the sums of the numbers in each region of neighboring squares. Those sums should all be the same (14 = 7 * 28 / 14 because there are 7 rows at 28 per row distributed over 14 regions).