I keep seeing posts talking about how AD carry and Mid are the hardest roles to play, but I usually find them as the only roles I play lol >.< My first good character was Fizz (mid) and after that Ezreal (AD carry). After playing as Ezreal, I started getting good with the AD carry kind of character. Trynd, Graves, etc. It's actually quite a simple role to fill. Depending on which character you're going for, you'll normall want AS, AD, crit, and lifesteal. (Armor Penetration to, but you usually get that last).
For Ashe, to my understanding, you want to go AS and crit chance. So two items you'll need are Phantom Dancer and Infinity Edge. So anyways, to the "how to play" part of my little post.
For AD carries, your job is to deal tons of damage with your basic attacks, and avoid taking damage. In the early game, you do this through poking (mystic shot ftw! :3) and because of the pokes, you should be able to farm in peace. It is vital to get CS, but then again, it's vital to get CS as nearly any character/role out there, so I don't stress on that too much. In team fights, your help turns the tide of fights. Prioritize targets and take them down one at a time. With tryndamere I had problems because my team would never initiate, which meant I couldn't get in there and use my crazy crit chance (just build him up with ghost blade and infinity edge, and he gets a crit hit nearly every single basic attack). I usually made the mistake of getting impatient and initiating myself, but thats a big NONO. AD carries are usually pretty squishy, so if a team fight is about to start, don't even think about initiating, (although poking from a safe distance is always acceptable!). Because the AD carry is so vital, you want to make sure you don't die. But your "vitalness" becomes worthlessness if you don't do anything in team fights, so you have to time running at the right point. If someone starts coming toward you, hit them with CC and start to back off. If they retarget someone else, turn around and start unloading onto them. Your basic attacks should do crazy damage, and this is what the AD carry is supposed to do.
Anyways, I hope that helps. GL HF!!! AD carries are really fun to play once you learn them!