I must admit, when i first started tabbing i used kabuki's thread as a starter for my approach and it is well worth the read through
When i arrange, (and i have no musical theory experience, am completely self taught), usually i'll know the song fairly well beforehand. I notice a lot of people start with either the bass line or the melody first, however i write everything from the go, bass/melody and second voices. Using the program Transcribe!, (which is a bloody amazing program), and guitar pro 6, usually i will determine the lowest bass note in the song. This in turn helps me decide if i need a capo or to detune. I normally don't detune out of natural tunings too much as in the past i've found sometimes it helps while sometimes it makes things soo much harder. So tunings like EBGDAE/ D#A#F#C#G#D#/ DAFCGD and so on, as well as drop tunings, are usually what i'll stick with. Sometimes for songs with many dissonant chords i'll try a tuning like EBFDAE.
With the song structure, usually my focus is on a full chord sound, dynamics like tempo change/ soft or loud playing and the like. Sometimes i'll decide to drop the original melody in a song an octave so that the correct bass note can be played, (this usually occurs when detuning), and sometimes i'll drop the melody an octave midsong, (due to it being too high up the neck), however this is tricky on deciding how the flow is affected but its normally just a feel thing.
When filling out the chords i normally dont add too much more than the original piece however sometimes i'll drag out the chord by arpegiating the rhthym notes to maintain the sound, or not depending on the dynamic of the song at that point.
It hard to give full detail on the approach, however creating tablature is a personal thing, influenced by musical preference and many other things. My tabs are usually not the easiest to play compared to some, and are not really tilted for beginners, but thats because i tab to my preference and skill level, (which constantly changes and skills improve with time). I like as much as possible to retain the original pieces impact, dynamics/ chord progressions and all. I also like to use harmonics for another dimension in pieces where applicable, and don't like to limit myself to only using the end of the fretboard, (ya know get some high notes in there/ put some feel into it! )
Another important thing to note is that i tab predominantly for myself, so that i can enjoy my favorite songs the way i want to play them, which also has huge impact on writing out tablature, (such as creating tabs too hard for beginners )
One last thing that i should add that i always, ALWAYS, do now is play, play and play my tabs to ensure that they are at least playable. Sometimes tabs will sit on my pc for months before im satisfied and post them. I still have tabs i haven't posted for like years because i think they're still too hard, or have a very specific way to play them.
Hopefully that sums it up