As per the title I would like to wire the horn through the washer button on the sierra stalk. Can anyone explain the correct wiring please?

You will need a relay as button will not take current of horn.

Are you using any of the wiper switch functions at the moment?

You dont need a relat my rear wash wipe button is my horn push. cant remember which wire I used get a voltmeter across the switch. Just 1 note mine does noy work unless the arme is in the wiper parked position. It gives the required voltage to the horn, Bear in mind it sent 12v to the washer pump previously

Is your horn a single terminal horn or a twin terminal horn? is your horn switch live switched or earth switched?

you dont need a relay BUT your switch will not last long.....

all you need to do is disconnect the washer pump live feed (i assume the washer pump is just a live feed and an earth) and connect to the horn live...hey presto!
HOWEVER!
early sierra's i believe used live switching i believe and later sierra's use earth switching, the above will only work on live switched ccts.
if your switch is earth switched then after you run a live to the horn you then take the other wire to the wire for the washers on your switch and connect it up........it is better to use a relay for the horn as they can draw a lot of current when activated especially if your trying to use a weedy column switch.