First I pick a unifying theme: a game mechanic like bleed or crit, a weapon, a feat, or a character to roleplay as.
Then I see all the feats that could work well with this theme, even if I go past the limit. If I pass the limit, I start trimming redundancies or the least effective feats (or focus further on a sub theme if needed). If I go under the limit, I freely pick my favourite QoL feats, or crafting feats if I have Int. After that, I try to fulfill the ability/score requirements at level 30. Then, I put lockpicking and hacking, and check the crafting requirements for the gear I wanna craft. At this point, if I have stealth and spare points I try to turn it into an oddity build (so I add pickpocket and persuasion). Personal preference: I always try to reach 105 mercantile as well, because the economy can be very harsh on hard+.
The order of feats is based in experience for the most part. Like, some feats are seriously needed ASAP like Expose Weakness on a melee build so that's locked at lvl 6. If I pick Opportunist at level 1, I usually like to pick Sure Step at lvl 2, and so on. Others can be moved further depending on demand like QoL feats, or feats that are circumstancial. Those feats that aren't nailed down are subject to change up until the moment when I pick them in game, because when I make the build I don't really know at what stage I'll be during each level.
Then when I have to pick specializations I tend to go with the cheaper ones, unless the unifying theme is a feat which happens to have an expensive spec, and then I have to bite the bullet.
Some DEX builds require extensive consultation of the AP cost reduction table. Sometimes if I'm short on skill points it can be useful to put more points in INT so I save skill points in hacking, crafting and mercantile. The amount of skills affected by DEX and INT is exorbitant compared to everything else, WILL doesn't count because you probably won't heavily invest in more than 2 psi schools after the nerf.