You must find a picture with the numbers series in each rows and columns. A number means how many adjacent black boxes they are. Each series of black boxes are separated by one or more blank boxes. You can specify a blank box by hitting the CRTL key.
At first, blacken and bleach the rows and columns where you have no choice.
Find the evident boxes. For exemple : in the first row, there is a serie of three black boxes in a row of five boxes. So, the third box is logically black.
Use information you have to complete the picross, do not forgot the blank boxes, it is also important than black boxes.
Complete the picross by combining the preceding technics.