When you’re cutting something odd-shaped into small squares, such as a potato or a chicken breast, first slice it lengthwise into the desired thickess. Stack these slices. Then cut lengthwise down through the slices to make a stack of long strips. Keep the strips lined up, then turn them perpendicular to you. Cut crosswise across the all the strips at once, producing small squares.
If you cut the slices the across the short length first, the smaller pieces scatter and you get an unevenly cut mess.