I am trying to write a track that has sustain strings and when I listen to it, it seems to me that the strings are delayed with respect to the rest of the orchestra. Strings have a slower attack and they sound as if they are not synchronized with other instruments, can you suggest a way to fix this? I know I must play them earlier so they can catch up with the other instruments, but how much earlier should they play to synchronize with a pluck sounding instrument like celesta?
Learning to program strings is like learning to play an instrument.
If it feels like they're playing late, then you have to play them early. The way to do this is literally to play them early. A bunch of samples won't all have the same attack so you will just have to play it until your hands get the idea your brain wants to hear.
If you want to use some kind of expression with that, then the best way to 'feel' it is to map something like mod wheel to the expression controller, so you can play it all in in one go.
If you click it all in from an editor & try to adjust later, it will always sound stiff.
One trick is to never play chords with strings - record each line separately, each with its own controller set.