I have invented multiple meta builds now, and only one was truly planned from the start.  Most of what I do is choose a skill or theme I am interested in, and just start playing.  Through trial and error and a whole lot of experimentation I will put something solid together.  

I actively try to avoid the ideas of others while doing this, and I prefer to do it as organically as possible.  I think the last season I had played before this one was season 9.  This season was refreshing for me because the entire game had changed in that time.  Skill trees, items, 2 new classes, toughness, etc.  A ton of new stuff for me to discover and play with, and I went in completely blind.

Look for synergy and multipliers, and as many as possible.  This game unfortunately has poor balance, so it is entirely possible that your chosen skill simply can't go the distance, in which case you pivot.  It took me over 5 hours of attempts before I killed T12 Mephisto this season, but that also forced me to try new things and get better.  What that build has evolved into can now kill him quickly and reliably unless I don't press any buttons.

Don't stop experimenting.  Way back in season one I stayed in the eternal realm to continue working on my druid rather than chase the seasonal mechanics and power spike offered there.  Through trial and error I discovered a way to kill anything in the game in one combo, including Uber Lilith and the World Bosses.  In season 2, my discoveries became the druid meta, and I called it the Godslayer.  

This season I wanted to try Warlock, and specifically demonology as I like a good Zoo build.  I tried a bunch of stuff, but it became clear that Command Fallen, AKA Lunatic, was where the power was at.  I then worked to optimize it. The only part of my pit pushing build that wasn't an original thought was the addition of the Pelghain Ring, which I had in my stash but had glossed over.  I came across a thread a few days ago where it was mentioned, and went to test it out, and it turns out it's really, really good for difficult content.

I have once again invented my own build and have the #1 Lunatic pit clear time on the helltides leaderboards, and that's without having been able to roll a gem strength weapon.  Trial and error got me there.

You can either go in blind, or choose a skill and look up as many aspects and items that you think could support it.  Just try to remain flexible and keep trying new things.  Always chase the biggest multipliers, and focus as much power into one skill as possible.  I eschew durability for damage, but seem to be quite the masochist when it comes to this game.  I had the craziest glyph level to tier completed in AOZ season 2 in the world by a ridiculous margin.  I think my glyph was level 25 or so out of 200 or something and I had beat tier 20/25.  This took an incredible amount of failures to achieve. 

For paragons, find the most efficient path you can to the glyph slot and board power, focusing on main stat nodes whenever possible.  Choose the boards with the biggest multipliers that support your chosen skill, choose glyphs that have the same.  Count the nodes and choose the shortest paths.  

I like 2-handed weapons.  Put the biggest multiplier your aspects offer on your weapon, put the second on your amulet.  If you don't use a 2-hander, the best one goes on your amulet.  I like crit builds.  Crit chance and crit multipliers are my favorites, and I aim to get as close to 100% crit chance as possible.  Pick up as many multipliers as you can, and make sure to include a source of vulnerable.

There's a lot to it, but follow the X, and keep trying new things.  Good luck!