STOP, You're Training Your Biceps Wrong! | ATHLEAN-X
While the title itself specifically mentions just the biceps, I think this is a great video overall on how to workout. My key takeaway from this video is how important the rest period is.
The rest period after working out is equally as important as the workout itself. The workout doesn't actually grow your muscles, rather it just provides the stimulus for growth. The growth itself occurs during the rest period. So, if your muscles are not getting enough rest, they will not grow.
^ This kinda reminds me of how important taking a break from reading is as well. If you don't give your mind a break from reading, you don't give your brain time to internalize and learn from all that you've read so far.
Oh and the best exercise to safely train your biceps, is the close grip underhand pulldown.