As all the locks in my place are years old and rusty as hell I have started replacing them with new door locks, so lets start with a privacy door lock, these are ones for the bathroom that don't need keys, ie anything or any key will open them (no pins in the tumblers or whatever).
So here we have the new lock set, this beasty was 190baht, don't bother with the cheaper lock sets in Thailand as when they break you have to damage your door to open them and doors and door frames are expensive, also stick to stainless, the brass ones look crap within a couple of months.
Now if you look at the lock in the above photo the left sided ring or bevel is threaded, this allows you some movement on the seating of the latch, ie you turn the knob and the latch fully opens, if it doesn't then you can't open the door.
In this photo you can see the secret button, this you need to depress to get the handle off and backplate.
Bottom right is the key for pushing down the secret button.
So, fit in your new striker plate not forgetting the lovely orange box behind it, next fit in your latch but only do the screws up most of the way, your going to need some play in it to slide the main section of the lock in, slide in the main lock and give it a few turns to make sure it operates the latch, back plate next and screw it up loosely first making sure everything still twists and turns, ie the lock works, if it does screw the back plate up fully and make any adjustments you need with the threaded left side beveled edge, if it all still works screw the latch in properly, stick the cover over the back plate and the other handle on, will need your key to press the secret button to allow the knob to slide on and away you go, shall change another lock tomorrow, probably
This video of changing a lock is pretty good although they have no back plate and the screws go through the whole lock.