First off, blocking a high kick effectively depends on how soon you recognize the kick and how it is being thrown, Chopping downward high kicks are defended against a little differently then one that is swinging upward and coming over or glancing off the shoulder and around your neck. Best practice for either if you recogfnize the kick in time is to step laterally and at angle toward the kicking leg \ , cover up high and lean at the waist into the kick with your elbows tucked tight into your side, chin tuck into your chest and glove pressed against the side of your head. Again, if you recognize the kick early enough this is the best practice because it will stop the kicker from getting any momentum into the kick, stuff it short and puts them off balance. When i do this i try to aim the point of my elbow (tucked into my body) at the midway point of their thigh, then step and lean right into the kicking leg.
Alternative methods if you dont see the kick in enough time to "stuff" it.
1. First off, stepping or leaning back is a terrible idea, unless they are really reaching for the kick. I would instead say to hold your ground and lean or bend at the waist, away from the kick (if you dont see it in time to stuff it). Basically making a bend at the waist and creating a plane with your upper body that the kick will glance off or slide over your forearm/shoulder. Again, keeping your chin tucked and your glove tight into the side of your head make the bend at the waist creating an angle with your upper body.... like / . Once you get proficient at this, and you can feel the kick beginning to slide up your forearm or shoulder, you can actually throw up your elbow (chicken wing) and toss the kick up over your head as it makes contact. Lean / , catch kick on shoulder/forearm, toss the elbow on the side catching the kick straight up in the air keeping your glove against your face (hence, "Chicken Wing")...... Do this effectively and time it well you can easily toss your opponent off balance (depending on how flexible they are), step in and smash your shin into the post leg as you toss the kicking leg up in the air. Once you get the timing down, this all becomes one motion and you will start watching your opponent doing mid air cartwheels when you bash your shin into that support leg after tossing the kicking leg up in the air:icon_twis