Why The Skill Order
This build is made for a Wisp + Melee Hero team in a top or bottom lane. An average game gets about 5-8 kills and a good deal of assists.
As you can see, I dont upgrade tether more than the first level due to the stun effect being so minimal. The stun only occurs once per tether, it is a mini-stun, and for the stun to happen you have to have the enemy come between you and your partner. Wisp is fragile and for a unit to get that close to him is normally dangerous, so thats why you dont want to happen.
*Also, you can use Tether on creeps or Heros to move through lanes fast due to the fact Wisp will pull himself to the unit.*
Spirits can be used two ways. 1) You can get close to creeps and use D to pull the creeps in, doing a fast 500 damage in 4 seconds like a melee unit. 2) Use the normal length or F to get early kills, or just harass mid-game. Wisp is one of the best harassers early to mid-game if you know the range of this skill and can follow an enemy in the perfect range.
Overcharge is how you will keep your HP a little injured at all times. Even if you just spawned, use Overcharge to get about 100 hp off of you, enough to the point that when you use Bottle, it will actually heal your partner. It also can do amazing speed boost effects to units who already have high attack speed, such as Anti-mage, for early kills.
Relocate is one of the weirdest to master ults. If you have a strong carry and arent getting too heavily ganked, you can warp to the fountain, heal and tether your carry, then warp back and get easy kills. You can also warp FROM fountain, to where an injured hero is running to, Tether him the last few seconds, and warp back, saving that hero's life. If you are laning, you can also use Relocate (near a tower for safe return) to another lane, Tether that unit and Overcharge him to push a tower or gank a unit. Just remember to untether him before you pull him all the way across the map to your lane.
Why The Item Build
I've seen the Wand + Urn build so many times and have yet to see it work to full potential. Most people cant keep an eye on it and use them effectively (nor can I at all times). I go with an early Headress and Bottle to give myself some easy Health Point Regeneration (HPR) and the bottle to heal your partner. This Headress will heal at all times, you can get extra tangos to heal HP, and your bottle will heal HP.
A Meka is a must. Wisp cant be fully used without a Meka. Tether a unit, make sure you are injured, and Meka. You will heal the unit twice. For the Arcane Boots, you can do the same thing to double up on the effect.
Vanguard is more of a personal item and keeps you from being an easy farm. Late game you will want to get more HPR items and Armor to protect Wisp as he needs to be able to get near (not in) a battle and safely get out with minimal damage. Meka will be able to save you for only 250 HP, enough to heal only 1 or 2 hits.