The next step in your practice is to move on to Beginning Stance. This form is an important foundation form for many of the forms to follow, so a fair amount of practice time should be devoted to developing a good Beginning Stance.
Make sure that as you get into this form that your stance is not too wide. One common error that is made in this stance is to step out too far into an uncomfortably wide stance. Remember the general rule of stepping that, when you step out, you should be able to bring your foot back in, while maintaining your balance on the weighted foot. Do not fall into your stance. If you do get into too wide a stance, then adjust until you feel comfortable. Never feel you have to stay in a stance if you are not comfortable, always adjust.
Remember to keep the toes slightly turned in. Only turn in as far as you feel comfortable up to 45°. How far you turn in the toes is not important at first. Anywhere from straight to 45° is fine as long as the toes are not turned out. You turn the toes in to open up the energy meridian in the back, so if they are turned out, you end up pinching the energy channel. Try turning your toes in then out to feel the difference. Take your time and only turn in as far as comfortable without undue stress on the knees.
When you do your side to side movement, move gently in a flat manner, not rising or sinking, not turning the waist. Make sure you stop before going too far to either side to avoid too much stress on your knees. Try not to think of moving straight side to side, but rather like turning around energetically before you reach an extreme to either side, as if moving in an invisible infinity symbol. Outside it looks like you are moving side to side, but inside you are following that infinity symbol.
The name of this form comes from what you are doing when you practice it. First you are in Preparation in order to prepare yourself to do Taichi. Then you separate yin and yang, substantial from insubstantial and begin to do Taichi. Remember the Taichi symbol. Taichi is separating clearly substantial from insubstantial, it is moving meditation and that is what you are beginning to do in this form.
You want to make sure that you devote a lot of practice to getting comfortable with this form. You will use it in many forms and practices to follow, so you want to be able to do the form comfortably with no effort. You want to be able to truly feel yin and yang clearly, and flow from side to side in a continuous, uninterrupted manner. You want your mind calm and concentrated as you float from side to side, keeping aware of both the substantial and insubstantial as they exchange throughout the movement.
Both this form and Preparation should be practiced as much as you can in the beginning. They both are essential to your progress as you advance through your different forms, so even though they are the first two, always devote time to them, no matter what your level of practice. All these fundamental forms are important to building a good solid foundation for your progress into higher levels of Taichi practice. Each day approach that practice as if you are starting all over again, and everything is new. This will serve you well in the years of practice to come.