Good day Athanasias!
Often, people make a confusion between the "objective" autority in the Church (for Roma, this autority is verbal tradition and bible), and the "directive" autority (for Roma: the Magistery and Pope), and imagine that by "sola scriptura", Protestants (among them: Reformed) mean
"myself alone with the Bible".
It would exlpain, of course, the great number of schisms and divisions you can see todays.
In fact, "sola scriptura" means that everything wich is necessary for salvation of your soul, worship of God and morality in your life, God made wright in the Bible's canon, outside from what you cannot establish a dogma, or another law in the worship. So, in the Protestant doctrine, the "objective" autority is not both verbal and biblical teachings, but only biblical, on the contrary to Roma and Constantinople which admit to stand a dogma upon not only scriptura, but also berbal teachings.
Now, we must see what is the "directive" authority in protestantism. Is it a infaillible magistery, like at Roma?
No, of course.
Here, that's the Holy Ghost, through scriptures (John 14. 25// Ephesians 6. 17, etc; ), Holy Ghost who is not in only one man who would be right a priori (= a guru!

), but whith all the Church (as long as she refers to Scripture), in everyone of her members (universal priesthood), so that if everyone Christian can read and understand Scripture, all of them can also do the same thing!
So, Scripture must be read commonly with all the Church, which commonly interprets it (thanks to the differents gifts gave by God --Eph 4. 11-14) without any other doctrinal spring than scripture (scriptura sacra sui ipsius interpres = Scripture interprets itself sufficiently).
Prudence and charity must be observed by everyone (Gal 5. 19// Jacq 3. 1// Eph 4. 15// Jacq 3. 13-18), so no one is allowed to profess a new doctrine (or contest an old) "just for fun", if you see what I mean... (= without serious and uncontestables reasons and demonstrations).
To sum up, "Sola Scriptura" means that Christians have to believe anly what God obviously gave to the Holy Churh for all the times by the mean of biblical canon (cf Luke 1. 1-4).
Every others things being not enough sure (imagine people who think that S Luke was the first to paint an icône! Build a dogma upon a such legend would be the shame and the ruin of the Church!).
It is what the first article of the Tetrapolitan's Confession said strongly, and what the 21st article of the Augsburg's confession said more implicitly.
PS: it doesn't mean that a non biblical tradition can't be observed at all in the Church. But in a such case, the caracter of this human-tradition must be claimed as being.... a human tradition (free, or established only for discipline), and (especially) MUST be not opposed with the biblical teaching.