First of all begin with CLOTH ARMOR. Then buy LONG SWORD and buy MADREDS RAZORS. Then buy WRIGLES LANTERN. Then you buy BERSEKER'S GREAVES and after this you start buying items for MADRED'S BLOODRAZOR (I buy RECURVE BOW first). Of course you need PHANTOM DANCER for the crit most and the movement speed and then INFINITY EDGE. After INFINITY EDGE you can buy BLOOD THIRSTER (you can sell the WRIGLES LANTER to buy it easier. If you do buy another PHANTOM DANCER)
With this build you can play both Jungler or Lane Udyr and he hes very good! You should better not change the order of the abilities because the build may be destroyed, but you can raise the BEAR instead of TURTLE (better for laning).