I think the key is in understanding what the composer did and why. To try to get into their shoes and follow their thinking path.
Depending on the composition, instrumentation and specific things you'd like to learn it make take different forms.
@ttw proposes copying scores, I'd suggest transcribing by ear and then comparing with a reference score. Yes, it takes lot's of time, but it's very engaging and forces you to think hard about what you hear rather than just follow mechanically.
Performing someone else's music is a simple way to internalize their style.
Copying various technical aspects, like specific instrumentation, use of specific instruments, giving certain roles to certain instruments, specific setup of instruments (e.g. tuning) is another way to get yourself an opportunity to think like the other composer.
Finally, analysis of every music element – from the overall form to motif structure. This allows you to isolate techniques used by the composer, so that you can try to apply them in the context of your own music.