If you have written a full ensemble score, MuseScore can create sheet music that shows only the individual part for each musician in the ensemble.
In the current version of MuseScore, the process of extracting parts from a full score involves two main steps:
You can define the parts at any point after creating a new score. You only need to define the parts once for each score, but you can make changes if needed. The following instructions use a string quartet as an example, but the same principles apply for any other ensemble.
You have now finished defining the parts. You do not need to do this again, unless you add or remove an instrument from your full score. In the current version of MuseScore, it is not possible to split a single staff (that contains two or more voices) into separate parts. So, any instrument that you want to print out a separate part for also needs to have its own staff in the full score.
You can create your parts at any time to see how they look, but it is only necessary to do it after you have finished the full score and want to print out the individual parts:
Repeat steps two and three for each part that you want to view or print. If you make any changes to the full score, you can re-create a part at any time in order to show the new changes in the part.
In the current version of MuseScore, changes on a score are not linked to parts that have already been created, so any change you make on the score will also need to be made on the individual part either manually, or by recreating the part from the steps above. Also, parts are created as not-yet-saved files, so each part you just created needs to be saved as an individual file (via File → Save...) or it will be lost when closing the tab. You may wish to review it before saving to check for proper formatting.