The bad news is that at a guess I’m going to say that cylinder is not UK legal, and if it’s legal where you are I would still be questionable about its safety
It’s a flat based metal (aluminium?) cylinder, which would have a maximum working pressure of 3000psi - approx 200bar
If correctly stamped it will have a maximum test pressure which will be beyond 3000psi - and must not be filled to that
The neck will have the standards markings stamped into it (assuming they are honest about it having met those standards)
Assuming everything is good:
The top of the cylinder will have a plastic cap, remove that and you should find threads inside
The regulator is upside down in your photo. The threaded end at the top of the photo should have some gaps in the threads - that is a safety feature so that if it somehow begins to unscrew while under pressure the air can escape safely before the regulator flies off
Screw that end into the cylinder
Do not use any thread locker - just screw it on
You should tighten it to a specified torque - if the documentation and markings are there then the cylinder and regulator should have a torque setting provided
If all is good then you fill it using the nipple under the yellow cap
The end that we can see has a pin in the centre. That is the pin valve which will release the air when in use
To use it for airsoft you will most likely need a remote airline with the appropriate connections on either end
Depending on the gun you will be using, and the output pressure of the regulator you will probably need another regulator to step down to the guns working pressure - check out the other threads on here about HPA systems
