Take a seat and ready your ears for the symphony of the collective butt clenches of all the builders. 

I've never playestested any of my builds.

...okay, not really. But planning was never really my style. I playtested the Curator once and Hands of Arkay twice. Since then I've not posted any (solo) builds, though I've played plenty of my own. Nowadays, I go in with zero expectations - race and sex are randomly chosen and I just play. I go with the flow, and whatever happens, happens. I have notes and docs full of builds I've never properly written up or, for that matter, properly playtested. I suppose that's why they were never posted. They can be replicated in form and function but not spirit.

But that didn't happen overnight. It used to be that I would spend hours picking through the Skyrim wiki and various UESP pages for scraps of lore, interesting weapons, skill combinations, and anything I could find to smash into a character that would use them all synergetically. And that's exactly what killed my motivation to play for quite a time. It was pretty much the root of my restartitis. 

So to answer your question - I used to meticulously plan out my builds and it was always the worst part of the experience for me. I playtested enough to know the mechanics but spent most of my time learning the character and developing roleplay. Now, I simply don't plan. I enjoy Skyrim more because of it but also never produce builds. I'm a roleplayer at heart and stripping away the organic qualities of the experience strips away the potential for the deepest, most enthralling roleplay opportunities there are. 

That said, the characters that have blossomed from my organic playthroughs could likely stand on their own as builds or at least act as the foundation for builds-to-be. So my method for building is...not really having a list of methods. Just one - wing it. It's a double-edged sword for the fact that the builds never get produced, but that's a reflection of me as a person and not the method.

Drawing is easier for me than writing so I usually start drawing concept arts for the character. Beffore I do that, I usually have an idea about character's personality and background( it is hard to draw a concept art of a character you dont know). I spend a good amount of time for the looks of the character, checking in game options, then mods. Maybe because of this, gameplay changes a lot during testing.  

When I get a graphic tablet, I will certainly try :D

When creating a new character, I ask myself what kind of in-game life I want him to go through, what will be his temper, phylosophy, psychologie, ect... Then I establish a rough line of event (quests, dungeon delvings, activities...), in accord with his personality the character likely is to do. 

From that point, I decide what will be his combat style, (favorit weapons, magic use - no/yes, what kind? - item consumption, craftsmanship...).

But I don't ask myself if his build will be efficient in game, sometimes it is awful to play because it suffers numerous flaws, in that I adapt my play style (or at least I try) to compense this weaknesses.

Often as not, it ends to stray from what I've planned at start. But I see it as the character evolving, his past experiences changing his mindset. 

Oh, one last think important in my plannings: mods. I try to find and implement all the possible mods that will make the experience with the charcter credible.  ^^

Why is this my line of process? I don't want to bother me with numbers and efficiency optimization, it tends to strangle in the cradle my new play through.

 I generally get an idea from some form of inspiration, which can be from a weapon from Skyrim (E.g Targe of the Blooded), to some lore (E.g Umaril). Generally I look into any related lore, type up a sketch of the build, a fairly rough one. Then I start the first playtest, complete it and see where to improve while adding onto the build. Then I start another playtest, using the new addition, while listing perks used, looking for glitchs etc then I simply do one last playthrough. If I remember I try to keep a save of the last playtest.