Both alone and in groups, with commentaries, and scholarly books and tools.
Wait, both means two, and that was more than two things! Forgive me for that!
I think nothing can replace devotional time with the Lord. It is our daily bread, and just reading, and reading the Word teaches us and helps us grow. The whole Word, Old and New Testaments, not just select parts.
And when we use other sources, or sit in groups, we do need to be discerning and test the spirits. That might mean going back to the Word, using a Concordance or other resource to check to see if the teaching is true or not. I learned an incredible amount about the Word in Seminary, although I had been a Christian who read the Bible through yearly for 25 years when I started.
Seminary taught me to really dig deep, and to question what others say. Not that everything I was taught was wrong, but the main thing I got from Seminary, besides ministry, was that as believers we all have to dig deep into God's Word, and not trust everything someone who proclaims themselves teacher, preacher or a scholar writes.