I won't say how much these door handles cost but lets just say they're expensive.
When fitting brass ironmongery like this you need to make sure you take your time and use the right tools for the job.
All screw holes need to be marked then piloted with a twist bit slightly smaller than the screw (in this case 3mm), brass screws are generally pretty soft and have a habit of braking or rounding off. Your screw driver also needs to be the right size for the slot and they should be tighten by hand, a drill or impact driver is too unwieldy for this type of ironmongery as a single slip can scratch it or wreck the screw.
I normally spray the mechanism with a little silicone lubricant before putting it together, as I figure it will be probably be the last time anyone has it apart in a long while and it should make operation smoother.
When fitted and all tighten up it should work with a positive action and if it's solid brass like this one then it should also look rather good!