During construction ensure that all components and wire links are flat on the board.

When fitting components with two leads, such as resistors, bend the leads outwards (only slightly) to stop them falling out of the board during soldering.
Don't bend the the leads flat as it makes them difficult to remove if needed.

Solder just one pin of the transistors and check that they are straight before soldering the other two.

Note that the flashing speed can be altered by changing the values of the capacitors.