As powertaco said get glyph of Rending to begin with immediately.
Priority is as follows, Rend - MS - SD/Execute - OP - Slam of you have nothing else to do. Arms DPS is GCD locked means any GCD wasted you loose DPS. DOnt clip rend DOT, let it fall off and reapply immediately. Arms excel in single target DPS, aoe fights and movement intensive fight will hurt you a bit but nothing to worry about.
Also arms warrior actually try to go for hard cap as its comparatively easier for them since they get 10% from stance and 5 % from t9 2pc. SO I would recomend gem arp to softcap first (53%) and then go for hard cap. As soon as you get to 75% Arp drop NES and try to get as close to 100% as you can.
Also you dont need to stack expertise, all you need is 18 with 2 points in weapon mastery (26 without it) or you can use a mace and drop it down further.