Thank you beforehand!
The most important skill lies between your ears: LOS management. You
need to develop a subconscious awareness of how to approach monsters
while minimizing their window of opportunity to detect you.
On the subject of spells and ranged attacks, conjurations are noisy.
(Sting and Magic Dart are pretty quiet, though.) That having been said,
when things do go sour and you've got a small mob after you, Mephitic
can work wonders. (If half the level is awake, a couple of Mephitic
Clouds exploding won't make things much worse.) But in general, stick
with enchantments (Confuse, Petrify, Slow, Hibernation) to facilitate
stabbing, and missile weapons instead of conjurations. Stick with
missiles that don't require a launcher until your Stealth gets decent --
the time needed to switch back to your blade is another turn that the
sleeping monster may notice you.
That's some good advice. What about god choice?
There aren't currently any gods suitable for a stealthy characters.
There are multiple proposals (check wiki:
https://crawl.develz.org/wiki/doku.php?id=dcss:brainstorm:god:start ,
note Thief and Stealth god pages are separate). But nothing definite.
Stealth isn't a playstyle per se, because only killing monsters yields
experience. The most popular approach is to focus on Stabbing. People
who don't mind being a bit cheesy suplement stabbing with the
hibernation spell and Mephitic Cloud. If you don't plan to use the
spells, have a backup plan or two.