Get there early and get someone o walk in with you and help push the cows. Once a horse figures out the cows move away, they usually get the drift pretty quick.
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This. I like for any young horse's first experience with cattle to be positive, so I introduce them to cattle that are a bit wary of horses. Cattle that will move away when you go toward them. There isn't much that can turn a young horse off of cattle quicker than to try them on hooky, fighting, soured cattle.
As for when you actually get into the pen with the cattle. I try to do more than just look at the numbers on them. I will study the cattle beforehand and try to remember certain features that go with certain numbers. It's hard to do that if they're using all black cattle, but if they are mixed herds (black, red, gray, mottled, Hereford, etc), then you can look at them and remember "Okay, number 4 is the big Hereford, number 7 is the small black with the longhorn blanket, number 5 is the red and white pinto colored one, number 1 is the black with one horn, etc". That way, you can just look for features that are easier to see from every angle instead of numbers that you can only see if you see it straight on. I've even been known to use poop-marks to identify an otherwise nondescript bovine LOL.
Beyond that, like another poster said. Slow is smooth and smooth is fast. It is much easier to sort cattle if you can walk into the herd, nudge the one you want, and walk them out of the pen. If you start pushing too hard, the cattle get riled and they want to bunch up. Then, you have hell getting them to separate off one at a time. When they're riled, it's all or none.
Most importantly, breathe and relax
. I know it can be a bit nerve racking, but most folks in the sorting sport are very welcoming and helpful. If you screw up (and you will, heck I been handling cattle all my life and I still screw up), just learn from it and go on. If you don't know what you did wrong, just ask someone and they should be able to help you see the mistake.
Oh, and PICTURES!