Here is the link to instructions for ubuntu and other Linux platforms- indilib.org/download.html
I was unclear what platform you are using but if you look here edu.kde.org/kstars/
there are general instructions too.
I'm not sure that you need the gsc any more and the last time I used this command it did not work so if you have this problem omit gsc from the commands (see below). Just type these into terminal.
This will lock in the ppa repository to be updated when you do a software update. The latest nightly build should show as Version 3.5.4 beta (find this in the "help" menu in "about KStars").
As the nightly build can introduce instabilities, once you find the setup works I would go into the "software update" settings in the Administration dropdowns and uncheck the indinightly updates and any stable updates in regard to KStars/indi. This is in the "other software tab." This will lock in the working version and avoid updating for a while until you gain confidence in the system.
I will send you my workflow that I tested a couple of days ago which worked well, I will have to send this from another computer.
Mike