Blocks can be connected in various ways, depending upon their function. How a block can be connected to another block can directly be seen from the block itself.
Block 1 and 2 have both an input and an output, block 3 only has an input. The blocks can be connected to each other by selecting an output (black circle) and then, while holding down the left mouse button, dragging the line that appears on the screen to a block with an input (black square). When a green square appears around the block to which you were dragging the line, you can release the left mouse button.