Tips to Use Undying (Dota2)

• One of the best skills to use early game is Decay: it's quite spammable, and will make ganking easier. You can easily get first blood if you're laning with a good partner.

• Decay is very useful even if left at level 1. Max it when team battles begin to start: you can spam it even more and make enemies squishier.

• Undying can be played in many ways: if you're going for a more supportive role, use your Tombstone and your Flesh Golem to slow the enemies while spamming Decay, then let your lane partner get the kill. Otherwise, use Soul Rip to finish a Hero while there are many creeps nearby: this way it will deal more damage.

• Don't use Soul Rip to harass, but use it to heal a friendly hero or to finish off enemies. This is more effective if there are many units nearby, such as creeps if you're laning, or heroes if you're initiating.
  • Remember that Soul Rip can be used to heal your tombstone.

• While initiating, use Tombstone and Flesh Golem, then Decay/Soul Rip: this way these skills will deal more damage.

• Though his spells are individually quite cheap, you will almost always have mana problems since both Soul Rip and especially Decay are very spammable. Consider getting Arcane Boots during the laning phase, then disassembling it once you have enough to buy the remaining pieces of a Bloodstone. You can then convert your standard boots to Phase Boots allowing you the mobility and chase power to keep Flesh Golem's powerful aura on the right targets during a teamfight.

• Another good item for Undying in the early game is Soul Ring. After using Soul Ring, you can cast decay for free. If you hit 2 or more Heros with this spell, you make up for the HP cost for Soul Ring.

• If you don't have enough money to buy a Bloodstone in time or you'd prefer cheaper items, try to make yourself as tanky as possible: if you're initiating, enemies will target you first. That's why you need an early Vanguard and/or a Hood of Defiance if the opposing team has a lot of nukers. Other useful items that increase your HP are Rod of Atos, which will probably solve most of your mana problems too, and Heart of Tarrasque.

• Should the enemy team have an Earthshaker, it is unwise to place a tombstone at the beginning of a teamfight. Each zombie will cause the echo to deal extra damage, likely obliterating your entire team (or at the very least severely injuring them).

• Zombies are magic immune, but will take full damage from AoE physical abilities. Be aware of heroes with these abilities that can easily deal with your Tombstone zombies, such as Gyrocopter's flak cannon, Axe's counter helix or Bristleback's quill spray.

• If Naga Siren is on your team, be aware that Tombstone will summon zombies even when the enemy targets are sleeping and untargetable. This can be used to your advantage

source: http://dota2.gamepedia.com/Undying