The basic concept is that there are nine rectangles in the grid. All the integers in the far-right column need to be the sum of all of the integers to the left of the column. All the integers in the bottom rows are the sum of the digits above it. Please make sure you don't pay any attention at all to the diagonals. Some students get fixated on that point and it ends up really slowing them down to a near crawl. You can always run a double check on your answer by subtracting the sums you have determined by the previous addend. You should get the starting addend as the difference.
If you are looking for a massive number of grid question look at the bottom of the page there are endless problems down there. Among the worksheets you will find a lesson, quiz, and review sheet.