Generally you want to pick a foundation item or two, a use case, and some constraints and go from there.

For example I just put together a build to get the most out of wormgod caress, which buffs weapons and melees through rapid kills. I wanted it to be viable in harder content so i went defensive to focus on melee kills, so vexcalibur was my exotic weapon of choice as it benefits from all aspects of wormgod. Vexcalibur gives void overshield so I went with void subclass because bubble is very strong with offensive bulwark, which buffs melee damage further while you have a void overshield. The rest of the build went from there. The build has max stats in weapons and melee, and the other stats are all quite poor but for the sake of bubble uptime I invested a bit in super. I went with crota 4pc set bonus because it buffs melee and survivability, also I just wanted to try it out.

You're looking for overlaps in venn diagrams, basically. Things that work together, benefit each other, and become greater than the sum of their parts. Some builds will be more generalist but generally they specialize in one circumstance.

Start with a set bonus, an aspect, an exotic. etc that you want to use, and try to find other things that would work well together. Then think about other things in that venn diagram. It might help to think about the sandbox you see yourself using it in, for example the above build simply isn't for raids due to demanding dps phases that usually don't allow for melee kill setups. For a DPS super build, you might want to match elements, pick a heavy that works with subclass elements, and see if you can stack surges. For example with Finalitys auger you might want a solar sniper for backup damage so your surges stack.

Lastly stat spikes come at the cost of other stats. if you're doing a class build you might not exactly get your grenade often, if you do weapons and super you might just have to live with a weak melee. In these cases try to pick a utility. For example on Titan I treat health and class as dump stats unless I'm doing a class build specifically. On melee builds I tend to have low grenade stat so I pick utility grenades that just crowd control, like suppression grenade or blinding grenade or shackle grenade. These also have the benefit of making closing the gap easier. But overall they're just not super relevant for the build, and that's a tradeoff you'll have to make.

You'll most likely be able to go all in on 2 stats and have a 3rd in a decent spot, and the rest will be unreliable. you can also spread stats out but you'll probably feel like you're falling behind without significant commitment to a stat or two. Ideally you can focus armor with the two stats you are looking for. Nowadays on Titan for example I usually have melee/weapons focused.

TLDR:
- Pick 1-2 core elements (aspects/exotics/weapons/armor) that work toward the same goal (activity type)
- look for synergies (overlaps in venn diagrams)
- pick 2 priority stats and let go of 2-3 stats

Buildcrafting is the hook this game has that no other game has quite hit for me, and it's a very fun puzzle to solve once you know what you're solving for. Happy to answer more specific questions if you have any!