You need to start out with a few pieces placed in rows as shown, with each piece overlapping the bottom third of the previous piece. I am using 2.5" squares and 6.5" rectangles but it could be 3" and 8" or 3.5" and 9.5" if you want larger pieces.
Then I add the 2 side squares for each cross.
The next row to the left starts with the top right corner of each rectangle matching the bottom left corner of the rectangle in the first row. Notice on the photos below that each vertical row is a sequence of 2 squares, a rectangle, 2 squares, a rectangle, etc.
Now add the 2 squares to finish those crosses. As you put each piece on the design wall, they should be just touching the fabric above them. You will not want any overlapping because when you decide to switch a color from one place to another you will have pieces fluttering to the floor if they overlap. Also when removing rows to sew you don't want to disturb the next pieces.
If you look at the base of the lightest cross you can see the pieces line up in a horizontal row, but the ones above and below are mismatched slightly. This is how it will look in the designing stage.
If you look at the pink cross nearest to the top of the quilt, you can see the side squares are not lined up perfectly when designing, but a little farther down the row it is matched perfectly. They will be perfectly matched when you sew it together. At this point look over the whole design and see if your colors and values are distributed evenly. 
On the upper edge I used 2.5" squares and 4.5" rectangles to fill in and make a straight edge. There are also some single squares in this row.
The lower edge is treated the same as the top with reduced size rectangles. If you look at the left edge, it is a 6.5" rectangle and one square with single squares filling in to make the edge even. Since I was using all scrap pieces of 2.5" strips, I only had enough to cut the edge filling crosses with some, and enough to make 2 full crosses with others.
I will be doing part 2 on how to take the pieces off the wall and sew them.