First up, we need identify where your screws are – this will help you find the joist.
Remove the old screws on the board that you are replacing, don't forget to wear your safety goggles and ear protection.
Once all the screws are removed, use a screwdriver to lever the old board up and out of the decking.
It's now time to measure the gap where your new board will be going and cut it to length. Place the old board over the new one and use your pencil to mark the correct length.
Once you have the markings, you can cut the new board using a circular saw or handsaw.
Place your board into the gap and now mark out where your screws will go. They should be in line with the existing nails on the deck, and 15mm in from each edge and 10mm from the ends.
With your board in place, pre-drill the holes with 2.5mm drill bit so the new nails won't split the board. Use a countersink drill bit it so that your screws sit flush with your deck.
Place the screw into the hole and then screw into place.