I improvise for hours then I transcribe... Painfully but happily if done right. Later, I transfer to finale and correct a few things. (I write chamber music mostly). If using Finale, however, I play back as many times as possible whatever I notated down until I get the sound I want. Usually I would hear some of the music previous notating it though most of the time I am wrong on what I think I notated right... good thing I correct it in time to get the perfect sound. (time consuming unfortunately)
So after some thinking and exploration I came with some theoretic solutions on how to notate on paper successfully without having the aid of a musical instrument. This treatise on the subject follows:
It is encourage to master 4 part writing and voice leading rules if one writes tonal music. This is crucial in a the sense that one see interval relation and the aesthetic that brings from writing in that form. ( and yes I mean no parallel 5ths!)
There are two ways to write music. You can visualize the piano for interval/chord recognition first before notating it on the staff.... Or you can visualize straight on the staffs without doing the mental parallel from the piano. I think the later should be aimed at. Why? Because one relies less on the piano.. thus saving time: you become used to visualizing things on treble and bass clef and line terms. instead.
So a way to improve on that could be to learn/play a piece on the piano visualizing the score in your head as you play it... closing your eyes... not so much worrying on whether you are playing the right keys or not, but hearing intervals and chords structures(open and close position) in your head instead. One can start with small simple pieces, original or not..<transposing those little piece in different keys is very! molto! tres! important..so that the brain can start recognizing patterns)
Speaking of patterns, it is suggested to create them . Practice ,for instance, just writing down arpeggios of a kind of chord ...say a major seventh triad.. several times. Visualizing and hearing it as you write them... say 10 times. After that,one can move to doing little motivic figures or musical ideas.. using maybe app., pt, nt, sus,... with some chromatism even... again all of this should be done several times and in different keys. It is suggested to sing them too ( only if you are 100% how they sound in your head) so writing patterns 10 times is good...why not 20 times :)
Lastly. Write down a note... any note and try to hear an oboe...or a clarinet...or a violin..or a horn( don't forget to do the proper transposition)Try to do this for 10 seconds everyday.
The ear should improve gradually hopefully if doing this exercises daily (that means less TV, play station, you tube, Facebook,YC off topic etc). There is room for improvement and an broader imagination to create more exercises of this kind... and though I am currently using the stravinskian way of writing music... I am at same time also doing this exercises with confidence it will enable me to write music using the mozartoviankovskian way.
my 2 cents...