The shoulder harnesses should already be fitted at this point, so just lap harnesses to go. There is only one hole on the far side of the car, but the inside/tunnel of the car has two holes, depending on the seat option that you have. I have carbon fibre tillett seats, so the correct hole is the rearmost one, which is parallel with the hole on the outside of the cockpit.
Start by using the bolt to clear out the threads. The order is the same as the shoulder harnesses, that is Bolt -> thick spacer washer -> harness plate -> wavy washer -> top hat spacer -> car body.
Remember that the red harness clip itself needs to be on the inside of the car to prevent people (like me!) from carelessly tossing it over the car body and damaging the paintwork on the outside of the car. By having this on the inside of the car this is less likely to happen, but it means you are using the harness box marked L/H on the RHS of the car and the harness box marked R/H on the LHS of the car.
The head of the bolt for the lap harnesses is very thin. Be careful when you are torquing this up, if your torque wrench slips off it will really fly, so make sure you are gently holding the torque wrench against the bolt and take your time. Torque to 47Nm, same as the shoulder harnesses.