Practice alternate picking a lot, slowly.
Practice legato a lot, slowly.
Then, practice alternating between alternate picking and legato as well--in the same line. Yngwie does that allllllllll the time. It's a huge part of his style. It's somewhat rare that guitarists actually blend legato and picking within the same fast run. Not unheard of, but not often done. ((at least not with conscious control! :P))
Then, study up on your scalar sequences. These are patterns set in a single scale that move diatonically up it--most of the time. For example, a classic Yngwie ascender in the Am scale is abcd...bcde...cdef...defg...efga. Notice how you are playing a 4 note pattern, but simply starting on different scale degrees everytime you do it. You could literally spend the rest of your life figuring out different scale sequences, but for Yngwie, stick to the classics--those are the ones he does. Learn them in phrygian, minor, and harmonic minor.
Vibrato--you have to work on your vibrato. Milk it like nothing else. Make it wiiiiide and slow. Make sure your intonation is perfect. Check yourself on a tuner to make sure. One of the most distinctive aspects of his style is that vibrato, underrated though it is. Listen to violin players as your guide for this--he did.
Arpeggios. Learn them up and down the fretboard. Just your classic maj/min/dim/aug. Sweep them, pick them, hammer them when you can.
All this should have you pretty much sounding like Yngwie pretty soon.
Back In Black isn't a song. It's a divine call that gets channeled through five righteous dudes every thousand years or so. That's why dragons and sea monsters don't exist anymore.
