As ban said it is all explained in the sticky at the top of the topic list.
If you mean you want 2 switches controlling one light then it's quite simple, but how you most easily or sensibly do it would depend on a few things. One common way of doing it would be to run the supply cable twin&earth (T&E) to the ceiling rose (Light fitting) and then run the switch wire cable (again T&E) to one of the switches. Then from there a triple&earth goes to the other switch. In reality there may be other factors which would make another method more suitable.
Such as:
Is this the only lighting circuit in the loft?
Does the supply cable pass near one of the switches on its way to the loft?
Is it just one lamp controlled by these 2 switches, and is it a normal rose/pendant?
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