It is hard as hell. Especially if you want to make it sound as close as possible to real orchestra. I found my self that this is tremendously difficult. After mix Im always thinking - ok, lets listen to it tomorrow - and when tomorrow comes, I listen to it and I know where to make adjustments in mix etc. And after that I leave it for next day and then if next day sound right... I do more adjastments :D And after few following days like this I can say it sound right, Im happy with it BUT again when I coming back after few days it sound horribly wrong. Never can make it right but always learn something from each mix.
This is simple 'black magic' for me. Especially when you think that different notes of the same instruments might have sweet-spot in a different place on EQ.. that just makes me headache!! Thats demand a lot of hard work, knowledge and practice!
Nah, when I'm saying that you can use orchestral instruments based on the idea of what frequencies are still weak I mean something staggeringly easy or even obvious like for example:
You have some beautiful highs with all the strings and great lows and even violas and celli seemingly fill the mids, but somehow the low mids have something missing.
You then know that horns saturate those lower mids perfectly and then use them.
Or the mids and upper mids could use more density then you'd go with oboe or clarinets.
Your bass is still in need of a boost so you go with the Tuba.
Sparkles in the highs are missing, so you add some glockenspiel and/or piccolo.
The idea is that you feel what parts of the spectrum should get something or even need something and then you go for the respective instruments that would want to live in these frequencies.
The next thought is in regards to transients, like whether you need harder attacks. You'd go for percussion like the timpani, or toms or any of the mallet based instruments like xylophone or marimba or the likes.
With synths you simply create the sound you want. With the orchestra you pick the instruments that fulfill that specific need.
It's important to be aware of the different possible articulations, of course, but it's particularly important to understand the nature of the instruments in regards to frequencies and possible transients.
Then it actually almost becomes easier, since it's a given set of options.
As for mastering, well, that's a whole different thing. Here, above all else, it's about what impression you wish to make, such as where does the orchestra play, is it meant to feel "real" or meant to drape around the listener in a more idealized way, depending on what situation you wish to let the listener be transported into.
And, of course, the general technical requirements for a proper listening experience, but that's really not the topic at this point. Would make a great topic, though! And I'm sure it's to be found all over the forum, hehe.