Levels 1-50
The key component of this build is Assemble Abomination. You won't get this until around 47-48 (depending on how many quest passives you get). But regardless, I recommend against doing this build until you have all 5 specialization slots anyway, as doing the abomination with 4 specialization slots is functional-but-suboptimal. Another perk for level 50 is the fact that your minimum skill level will hit 7 here, which is perfect for Infernal Shade.
You can feel free to do any build you want until you hit 50. This section will give one option aimed to transition as smoothly as possible into it. Reference the "Leveling" variant in the "Build Variants" section for a more detailed view of the leveling skill trees and passives (also linked here).
Hardcore Tips
If you choose to play this leveling variant, you should note that it doesn't have the same power level as the abomination does when it's online; your minions are more frail, and your front line will get broken at times. If you are playing on hardcore, make liberal use of the 'A' command to send minions in vs. faceplanting yourself, as well as to move them out of big damage (such as the avalanches in the ice section of the story). Note that it is okay to move backwards and summon your minions in the frontline. Use 'A' to target down the biggest threat that is killing your minion line if you end up in this situation.
Most importantly, if it looks critically bleak, 'T' to teleport out. Not worth the character loss to push through an incredibly dangerous spot.
Level 1-10
Progress through this early range using Marrow Shards as your primary DPS skill (or Hungering Souls if you're HC). By the time you reach level 10, you'll want this kit:
Game plan here is to hide behind Summon Skeleton and Summon Bone Golem (and keeping them all summoned). Keep Bone Curse up, and Transplant forward. Marrow Shards when you have nothing else to do.
Note that Summon Bone Golem has a huge mana cost; do not cast it recklessly and find yourself locked out of casting for several seconds in critical moments.
Level 16
Swap in Summon Volatile Zombie for Marrow Shards. If you gain access to Summon Wraith before zombie, you can swap that in for marrow shards, and then swap zombie in for wraith once you get it.
Summon Volatile Zombie fills the same filler slot here; our game plan does not change, and the priority is to keep our other stuff up.
Level 20 and 5 points in Necromancer
Once level 20 is hit, specialize Summon Skeletal Mage (if you don't have 5 points in necromancer by this point somehow, you can very temporarily specialize Summon Volatile Zombie instead, and then specialize mage once you unlock it).
The skill tree plan is to rush for Death's Cavalry 3/3; since we don't have our abomination yet, we don't need to prioritize the Cryomancers and Pyromancers nodes.
You will not put Summon Skeletal Mage in your kit yet.
We are waiting for Summon Skeletal Mage to get to level 7, at which point we get Grave Passage 1/1 and it will replace Transplant. Having said that, if you really want to, feel free to swap it in early (either for transplant and accept you have no movement skill for awhile, or for zombie and accept you have no filler skill for awhile, but once Grave Passage is unlocked, Transplant should be the skill that is swapped out).
Level 35
At this point, we will specialize Summon Volatile Zombie, giving us substantial ward generation in Vital Ward 1/1, as well as significant DPS increases through the Ravenous 2/2 and Grave Attunement 3/3 nodes.
Another option, if preferred, is to specialize Summon Bone Golem for a better tank in your party. Grab Tower of Bones 4/4 if you do this to keep aggro off you + your squishy minions.
Level 50+
Level 50
When we hit level 50, we will do a large re-spec. But, before that, what we're going to do is to immediately put Dread Shade into our newly-acquired 5th skill specialization slot. We are going to let that hit level 8 before we re-spec; this should not take long, and will make for a more seamless transition.
Once ready to re-spec, If you were following from the above build, we will be giving up our specialization in Summon Volatile Zombie and Summon Skeletal Mage, and taking specializations in Assemble Abomination and Infernal Shade. We will also want to start steering Bone Curse towards the Merciless node (you can re-spec out of Marrow Thief and Defile Defenses to make this faster, if you want).
During these early levels, your personal damage will be high enough that Infernal Shade is doing a non-negligible about of damage to your abomination, but, if sensibly geared and specialized, it should not be a danger to it.
Please see the "Why Is My Abomination Dying" FAQ section if your abomination is dying to Infernal Shade.
Level 58
Nothing conceptually changes at level 58, but the default build does get better at this point, as it gives us a minimum skill level of 8, which allows us to get both Cryomancers 1/1 and Pyromancers 1/1 when we spec into Summon Skeletal Mage, as well as get Burn Trail 1/1 when we spec back into Infernal Shade. You should re-summon your abomination when you hit this milestone for the extra buff from the 6th minion type.