Solder angled header pins at the servo port as shown in the diagram below:

Solder 5x5mm buttons SW1 and SW2

Solder optoisolator at U2

Solder JST PH angled connectors at J1, J5, J8, and J9

JST PH straight connectors at J3, J4, J7, and isolated output

Insert IC on top of IC socket at U1

Solder LEDs from D1 to D6

Solder 3.5mm headphone jack at J2

Solder 2.2uF capacitor at C1