The way tablature works is actually more simple than that. You don't need to even think about the key. Just directly transcribe the note you play to a note on tab. eg if you play 5th fret on the 2nd string, just transcribe that.
The only question you need to ask yourself is where on the neck you wish to play a particular note, and this will usually depend on how you want to tie the previous and following notes together - for speed you'll place them in a handy place to move your fingers to.
In your example, if the song is in C major, knowing how C major works on a guitar neck will give you a good insight as to what chord shapes will crop up, but you don't need that knowledge to just copy your finger position to a number on the tab.