Hi,
We sent our first two off last week, we tagged them in the afternoon and took them over in the evening as we were told they might tear them rubbing against trees.
We used metal Y tags (I believe they are the only ones suitable for slaughter) and borrowed a pair of tagging pliers from a neighbour.
I think the best advice I was given is practice on a piece of card first, it really helped me to realise how far the plier handles went together 'cos I was expecting them to close fully and they didn't.
It was much easier that I was expecting and only a very little blood on one of the three.
Good Luck