Before you start - NACA5 has ridge on one edge. Designate this conductor POSITIVE and ensure that, in every instance, this ridged side of the cable is connected to the positive (RED) socket.
NEVER solder speaker cables that are still connected to your amplifier.
It will help to have some form of jig or arrangement to hold the cable steady and locate the 4mm plugs while soldering and during the cooling period.
Either a couple of 4mm sockets in a plank of wood or just two 4mm holes (with the correct spacing). You should arrange a way of ensuring that the weight and stiffness of the cable does not move the solder joint while it is cooling.
N.B. in light of the additional advice below - you may wish to use two groups of two sockets/4mm holes.
MARK these +/- or RED/BLACK to reflect the socket arrangement on the back of your amplifier.
On Naim amplifiers (other than a pair of NAP 135s) - these are mirrored (take a look).
Good and sufficiently powerful soldering iron - pre-tin tip, apply to pin with cable firmly and neatly in place - feed in solder gradually, ensuring that the join is good and with no excess of solder above the pin.
Allow to cool without any relative movement between cable and plug (that's why you are using the pin holders and a clamp or similar - NACA5 can get very hot during soldering).
For best results - the pin should be free to wobble a bit in the re-assembled plastic holder so avoid using more solder than is required to fill the slot and bind the cable. This allows the plug to 'mate' properly with the socket - rather than being forced into a limited, two-point contact.
When soldering speaker cables you should first lay out the cable with the two amplifier ends (arrows pointing away from this end) together.
(It is a common error to take the entire length of cable and fold it in two to get the two correct lengths. A sensible move but this will, without correction, result in two cables that 'run' in different directions.
Dealing with the amplifier ends -
look at the rear speaker sockets on your amplifier.
On Naim units (Apart from NAP 135s) these are "mirrored". THINK.
When you replace the plastic case - will the correct (+,-) pins fit in the correct socket without resorting to contortion?
Get this right now.
At the speaker end - the sockets are likely to be UN-mirrored. Once again (and during life in general) THINK.
Imagine the cables coming up to their respective sockets.