I had to re-solve it from scratch, didn't take more than 20s (I made this over 5 years ago, while I can't even remember what I had for lunch yesterday...)
You start from the same grid size and green square as every other output. Then you stack the red blocks provided from top to bottom. There's a correspondence between where the last block ends and where the next block starts, which is provided by the training examples. The angle-shaped blocks restart from the lowermost square of the last block, but the lines start *below* that square.
This is really not hard IMO. It's slightly tedious though.