Hi, @1judeyns
Welcome to the forum.
Can I suggest you convert 12 to 5V and feed the UNO at its gnd and 5V pin.
That way you bypass the inefficient linear 5V regulator.
Tom.... 
How much current does the horn draw? What size battery?
Will there not be the risk with an Uno or Mega that that 5V will back feed into the USB when connected to a PC? A Nano would be a better choice from that perspective.
I would use an automotive horn relay here, switched with a transistor.
hi thanks for the reply. the current for the horn is 5A😁
You may find the turn on of the air horn draws a LOT more current than that.
You will need to be careful with your wiring to avoid the transient dripping the voltage to the arduino.
A UNO is very much an experimental tool - so I'm with @sterretje better use a nano.
Not sure why you want to drive the air horn from a microcontroller though.
The setup you have in post #1 is the safest way to go.
The buck will provide isolation of voltage transients caused by the horn. There is also no problem of having the USB and 8V connected at the same time
I would not power the Uno from the 5V Output pin unless you understand the consequences of doing so.
so my recent attachment is good to go? or do i need some safety components to prevent voltage and current spike such as pullback diode, resistor, capasitor, fuse etc? or it just ookay
why did u more prefer arduino nano? whats the benefits and difference between uno? is it because it has built in pullback diode or fuse or etc?
A fuse is always a good idea.
Put one on the poritive terminal of the battery.
The size will depend on how much current the horn uses.
what if i dont use it is it okay
from my recent diagram, which part of connection should i put that fuse? and do i need to put flyback diode? is it compulsory?
Like I said the positive terminal of the battery
and do i need to put flyback diode? is it compulsory?
No
thanks in advance sir! May God bless you
Thank you
Have a nice day!