Well, when I create a team I have one central thing in mind... don't create anything bog-standard. If I wanted a standard team I would just copy paste something and go with that but I think that's really boring. So avoiding too common things, at least to a certain extent, is my main priority with creating teams - this doesn't mean I will never use common pokemon, as you are about to see if you scroll down, but I always add my own touch to the team. Some people create teams the way they do because they know they'll be very powerful whether in general or against a certain opponent they know they'll face, I could never do that.
I want something that challenges me to make the most out of great potential but which will generally not stand up to your average standard team in a slugfight. This is because to me, winning itself is pointless - it's a good game that counts, but I still aim to win and don't just design teams like a kid picking candy.
I never create teams that try to cover everything. Rather, I want to make sure that my team covers itself - ie. that it's really a team that always works well together and not just a bunch of strong pokemon hoping to face the right enemy team. Pokemon that perform well in the role I give them or even exceed my expectations and fulfill more than what I intended for them to get placed in my mental registry of good pokemon.
Then I think of what I want this team to do. Take my most stable team for example, it consists of:
Forretress
1. Gyro Ball - SR - Spikes - Rapid Spin
Relaxed 252/252 hp & def, Leftovers
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Vaporeon
2. Surf - Heal Bell - Wish - Roar
Bold 252/252 def & sp. def, Leftovers
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Machamp
3. Dynamicpunch - Stone Edge - Rest - Light Screen
Careful 60/192/252 hp/def/sp. def, No Guard, Leftovers
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Rotom-w
4. Shadow Ball - Hydro Pump - Confuse Ray - Thunder Wave
Calm 252/108/150 hp/sp. atk/sp. def, Leftovers
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Smeargle
5. Spore - Substitute - Belly Drum - Baton Pass
Jolly 252/252 hp & spe, Own Tempo, Leftovers
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Dragonite
6. Agility - Dragon Claw - Fire Punch - Earthquake
Adamant 248/184/78 hp/atk/def, Leftovers
The pokemon I started with was my favorite, Dragonite. I knew I wanted him to be the final sweeper but I didn't want to use DD so I decided to Smeargle-pass. From here on, I had four team slots to use which I considered a separate team; these four slots should be completely independant from the last two. The objective of these four team slots would be to wear out, scout and semi-stall my opponent's team as much as possible.
With that in mind I put Forretress as my lead, because I wanted a rapid spinning double-spiker. Forretress's only flaw is its inability to deal with leads such as MixNite, otherwise it works great - especially against stuff like Metagross that can't boost one of its offensive stats while Forretress is spiking.
Next followed a lot of trial and error. I originally used to have Spiritomb there, but then I decided to put in Rotom-w which I am happier with. I did this because I thought Rotom-w would simply work better. The ghost is there to spin block and be a catch-all lockdown against sweepers.
Vaporeon was added to give longevity and sustainability to the four slots. I wanted something that could help me scout which is additionally useful because of Spikes, and I wanted Heal Bell & Wish to support Rotom-w & Machamp.
Machamp is there to give physical punch as well as help me soak up special hits. With 295 sp. def and 336 hp + Leftovers before Light Screen, Machamp is very good at taking special hits. After he's done he'll Rest, I'll switch to Vaporeon who will Heal Bell & Wish, then switch back to Machamp for another go. I figured that 296 attack doesn't really need to be boosted because Machamp's primary purpose must stay to with the 4-slot plan, and that plan is not one of offense.
Dragonite can in emergencies be used as a cleaner which is helped a lot by Forretress. I want to use Dragonite as a cleaner only as my last resort and never voluntarily switch into either Smeargle or Dragonite unless the time has come. Agility helps me outspeed things that a single Dragon Dance could never do unless I'd switch EVs, which would make him much more vulnerable to priority moves and attacks in general, which is not good because I always expect to take a few hits. Expecting to stat-up without getting damaged in the process is just foolish.
Of course, my thoughts are more spontaneous and anarchic when I actually make the team but I hope I provided some insight. That was for that team. Otherwise I usually throw random pokemon together with a vague plan that I've taken a liking to, test it out, make changes and go like that.