First things first. The most important part is knowing what you want. Your vision for the final version of the build. And then focusing on a select few pets and skills to do the job. You cannot afford to spread yourself thin or no items will be enough to save you. So find what you like and find the things that synergize with that.
Now, generally speaking you want to aim for a minimum of +1000% pet dmg, +40% OA, +100% Crit and as much Attack, Cast & Total Speed as you can get.
But that is not the full story. Obviously your pets will need enough HP and resistances to survive first before they can deal damage. That said, you could make a transmuted Blightfiend build where you use your pets as suicide bombers and hence don’t care for their HP or resistances.
Then there is Resistance Reduction and Damage Conversion. Usually you want all your pets to deal a single type of damage that you can build enough RR for. Elemental, pure Lightning and Acid/poison are good candidates for that.
You can also go for Bleed or Vitality/Chaos, but those require a bit more knowledge on how stuff works and for Pets, Vitality and Chaos RR isn’t exactly easy to come by. You also cannot convert a damage type into Bleed.
Then ofcourse, there is flat damage, like what you get from Hellfire aura for example. Obviously the value of flat damage is determined by how many pets you have and how much RR you have for the damage type and how much +% dmg your pets have. For this reason, Skeletons love flat dmg.
But it doesn’t stop there, for we still have the Summoner or player character to worry about. No matter how powerful your pets are, you still need to worry about your defenses. DA, Physical Resistance & General Resistances will be given priority.
2700 DA is the minimum for comfort when it comes to endgame content. Anything below 2600 and you will have problems. 2900+ is my sweet spot though
Pet builds do not really enjoy the luxury of stacking Armor or Armor absorption for that matter. Instead we get more physical resistance compared to the rest. So make use of it. Get as much of it as you can. But that said, don’t focus too much on it and lose sight of everything else. Also note that while it will help make you survive better, it won’t make you immortal as you still need to worry about other kinds of damage.
And that is where the general resistances come in. Getting them all to atleast 80% is non negotiable. But enemies also come with their own RR which reduce your resistances. So plan for that.
Elemental and Acid/Poison would be the easiest to Overcap (ie, more than 80%) given Occultist has Familiar’s Aura + Aspect of the guardian.
You will probably get enough Bleed resistance from items + components. Same goes for Vitality resistance. But Pierce may be a bit hard to come by although, with the buff to Spellscorched plating, it should no longer be a problem. And Seal of Might on both Weapon + Off-hand gives Bleed, Pierce, Vitality and Physical resistance for both you and your pets, making it an attractive choice.
Aether and Chaos resistances will probably be the hardest to come by and will require some management from you to balance out. Early on, when Seal of Might is not available, Purified Salt and Imbued Silver will be popular choices. Also, don’t expect to have too much of an overcap on either, even at endgame.
After that comes damage mitigation. Like Fiendflesh Greaves proc, or Seal of Ancestry. Perhaps Giants Blood or Tree of Life, etc. Remember that playing a pet build means you have no way of regaining Health other than potions, skills like Blood of Dreeg and Passive Regen.
And another thing to note is Move Speed. You want as close to 135% as possible for you will be kiting a lot against tougher enemies and you can’t afford to do that at a snail’s pace. It is also for this reason that Slow resistance can come in handy if you can get it without sacrificing much.
Other 2 things are Stun and Trap Resistances. Of those, Stun resistance will be the easiest to get and stack. Both will help prevent a lot of unnecessary deaths, especially in Shattered Realms, if you plan on climbing its higher shards.
And ofcourse, no Pet build is complete without %CDR, which stands for CoolDown Reduction. You ideally want as much of it as possible as it helps both offensively and defensively.
A bit related to the above, you also want a few Devotion Procs to assign to your pets. They will add some extra Dmg, AoE or even CC. And ofcourse, RR too. And %CDR reduces their CD too.
Then after that, it is just a matter of positioning and knowing your enemies.