No formula. No set style.
No one type of drone.
Yes, in terms of creative process "no formula".
But i can hardly imagine that there is any music label who wants to release unprofessionaly (or bad sounding) recorded music. All ambient music i listened has something common in quality of sound (in drone ambient genre too). So, it somehow reminds of "80s pop sound with same peresets used" 
I would advocate creating sounds you are happy with, you can try putting out tracks on bandcamp etc and see how people respond to them.
I used to use soundcloud too - where people could (I assume they still can) add comments to the tracks they are listening to.
You could also try sending a CDs worth of music to labels you think might be interested and ask for constructive feedback.
This will help you clarify the balance you wish to hold between what you are wanting to create and what other people want to listen to.
There is a very wide range of music / sounds created by artists in 'ambient' music - including those on this forum (let alone digging into the many other forums out there)....listen to different pieces and see how the range strikes you.
What artists inspire you? do you want to create in a similar vein or use those sounds as a springboard for your own themes and sounds.
Sometimes putting a very complex piece of sound through a variety of effects including the perennial reverb can create a backdrop / drone / atmosphere in which different frequencies and textures waft in and out - perhaps you could muddy things up further and see what arises?
