A neatly done running braid will be ok if you are at a schooling show. Unless you have a breed of horse that is accepted for their flowing long mane which are braided in a running braid by some (such as a Friesian or an Andalusian), I do suggest you trim the mane and braid it properly if you plan on doing more serious showing.
Just google running braid and you'll find plenty of pics and videos. I suggest you do a braid that is as close to the neckline as possible as you do not want to take away from your horses conformation and neckline while showing.
Rosettes rosettes rosettes all the way, unless you have a horse that is supposed to be shown (in breed classes I mean) with an untrimmed, flowing mane, and then you just do whatever you can make look good - but they MUST be braided in some form!