I say after watching your friend, if you have a horse that you don't show, clip that one for practice. :) Also clip before the coats change over so that if you really mess it up, it will be gone as the summer coat comes in.
Sorry, I don't know of any websites (though I am sure they are out there :) ).
Yeah good question, are you simply talking about bridle path etc or a full body clip?
Google is the answer.. be cautious of small nicks etc, and be sure to brush or preferably hose the horse off after being clipped, so that any small hairs that have lodged into tiny nicks can be removed. If they stay in there they can cause small swellings and get infected.
Also tip to give pony a baby oil bath after you have clipped him, and don't hose off the baby oil just rug up in an old rug so all dirt comes to the surface of the ponies' hair, cleaning him up real good and giving him a lovely shine after you have brushed him. :)
2Gelldings, I have faith in you, it it just pretty easy to cause lines when clipping. :) I would love to hear tecqunique tips to reduce the occurance of lines. I did well when clipping Dez this year, but had a hard time at the flanks, where the coat direction changes. :(