This guide is not gonna teach you how to do 1990s step by step. I think most people would agree that 1990s are a b*tch to pick up. There are so many things that can go wrong while you're doing the move, so I'm gonna list some of those problems and try to correct some of your mistakes.
Balance
Balance plays a BIG role in learning this move. First mistake... don't mix up static balance with moving balance. Handstands might help you get used to being upside down, but the feeling has almost nothing to do with spinning around in 1990s. I still can't do handstands for sh*t (especially one-handed) but I can do 1990s just fine. So don't waste too much time on those. Balance is the first thing you should worry about as you kick your legs up and bring your hands down. Beginners shouldn't put power into your spin until you can feel yourself "locking" into a balanced position that will set you up for a nice spin, otherwise you will spin out of control since you're off balance to start with. How can you tell that you're balanced? Experience. It's just a feeling you'll have once you get used to it, so just practice. Balance is also what gets you more spins, speed matters too but not as much.
Body
Your body should be locked up stiff during the spin. Don't expect to get into position and let momentum carry yourself through the rest of the move, 1990s rarely work like that. Even when it does, you won't stay up there for long. Keep your body STRAIGHT. You really have to pay a lotta attention to this when you're doing the move. Many people tend to bend back a lot, making their whole body look like a skinny C. Even when you think you're straight, record your 1990s and watch it. Chances are you are still bent. This bent form is very unstable, and, not to mention, very ugly. (If anything, bend the other way, stick your ass out and bend your knee a little so you look like a lightning bolt. This form is actually quite advanced, you need a lot of control to do it.) Bending your body is very natural, it will have a tendency to do so in order to stay balanced. You'll just have to fight this instinct by paying close attention to your whole body.
Legs
Legs are hard to manage when you're just starting out, they just seem to fly everywhere and lose control of themselves. Again, keep them stiff and pay close attention to them. Keep them in a good position and make sure they stay there by locking up your muscles. Keep them straight (don't have to lock knees) and stick them close together, it makes your spin faster and easier. (When I say keep them together, I mean keep your feet together. you can do different things with your knees when doing variations) If your legs are straight, you can just cross them at the shin. DON'T cross legs at the knee, your feet will come apart when you spin. You also mess up your momentum when you do that. Since your spinning radius is so small, it's easy to lose your balance. You'll just have to live with that since that's what 90s look like. A common mistake I see in people is that they tend to keep one leg straight and the other leg bent at the knee to propel themselves around. This may work from time to time, but it's a bad habit. You don't want your 90s to look like that.
Arms
Arms need to be kept pretty straight and also stiffened during the spin. Chances are, you have come pretty close to landing on your head during the learning process. This is a result of a loose spinning arm. NEVER lock your elbow, you might get off balance during the spin and it will hurt really bad. You probably won't be able to do 90s or handstands for a while when that happens. The other arm can remain near the ground just to catch yourself when you start getting off balance. If you're confident about your balance, tuck that elbow (free arm) in your rib cage and bend your arm so that your free hand is right next to your shoulder. This looks way better when you're spinning. Another thing about arms... your shoulders need to remain somewhat parallel to the ground. Big mistake is to lean too far to one side since 1990s are only on one arm. Pay attention to this. Once you're in your spin, you need to place all your weight onto that shoulder that's supporting you. Keeping that arm/shoulder stiff will help you maintain your balance.
Hands
Hands.. spin on the part that you feel most comfortable with, don't spin with your whole hand. The least amount of friction is usually on the bottom part of your palm directly below the pinky. This will help you spin longer. Placement of the hands is also very important. Experiment with different spots on the floor to put your hands down as you kick your legs up to get upside down. This gives you balance right away if you find the right spot. (This "sweet" spot varies depending on the power of the first kick and your speed) When you place your second hand down to transfer weight, place it somewhere close to the other hand. This way, when you transfer your weight, then center of your spin is remains somewhat the same from a bird's eye view. (Lotta physics involved)
Hip
The twist in your hip is what starts the spin. In 1990s, you kick up to get upside down, your legs begin spinning from the kick (or maybe an additional kick), then your body catches up and you're fully spinning. Make sure you don't let your hip twist too early or too late. Need to experiment with the timing of this twist so you can get as many spins as you can. If you spin too early or too late, you can lose speed and/or balance. Once you're in a full spin and your body is straight, your hip should be propelling your spin. If your upper body is bent, you will probably use your head to propel your spin by leaning your head toward the spinning arm. (You don't really propel your spin with these body parts, they're just what keeps the spin going longer)
Head
Not much to say here, beginners look down at the floor to get your sense of balance. When you have a good feel for it, don't look down, just look to the side and keep an eye on an imaginary circle on the floor. This helps keep your body straight. When you look down, it's highly likely that your upper body will bend just so you can see the ground right below you.
Injuries
I have only been injured twice doing 1990s. Once when my elbow locked up and the other time when I did a really hard landing on marble floor and bruised a bone on my foot. Don't worry about other parts of your body... as ridiculous as it sounds I've heard about Bboys wearing cups for 1990s. (No offense to those that do I just don't find that necessary) It helps to always fall with your belly facing the ground. I know a lotta people have this cat instinct in them so you don't have to worry. If you fall on your back you will probably get the wind knocked out of you. In that case, catch yourself with your feet. When you practice on carpet, don't do it too much in a short period of time or your palm will get carpet burnt... which isn't a big deal since your palm has a really thick layer of skin, but it does hinder the learning process.
Video Clip
Oh yeah, here's a small clip to go with the guide.
Scissor Kick 90s - I prefer to start this way, wind-up looks more impressive and goes nicely with combos. I can't even remember how to start 90s the normal way after learning this variation.
Intermediate guide for 1990s
Linear Mode
