练习核心姿势前先热身。 站在水里拉伸颈部、手臂、臀部以及腿部。 先在泳池里做出核心姿势。 双脚蹬墙数次，练习核心姿势。

Given the above summary, write a detailed text in English for it.
Move around in the water for about five minutes before swimming the breaststroke.  You could do another swimming stroke you’re familiar with (like the backstroke or freestyle).  This will help your muscles warm up and help your body adjust to the water temperature. If you don’t know another swimming technique, walk along the bottom of the pool in a section where the water reaches your shoulders.  Alternately move your right and left arms out in front of you in clawing motions, pulling water toward you. There are many ways to stretch these muscles.  For instance, stand on one foot and pull your heel toward your butt.  Hold the position for 10 to 15 seconds, then switch to your opposite foot.  Repeat on each side three times.  Stretch your arms by knitting your fingers together, then pushing them straight out in front of you with the palms facing out.  Hold the position for 15 seconds, then bring your arms up above your head and hold for another 15 seconds. Rotate your hips clockwise for 10 seconds, then rotate them in the opposite direction for another 10 seconds.  Repeat three times, then do the same thing for your neck. Place yourself laterally in the water with your arms extended straight in front of you and your legs behind you.  Your toes should point toward the wall behind you, and your fingers should point toward the wall ahead.  With your palms down, splay your thumbs out so that they barely touch.  The space between your thumbs and index fingers should form a ‘V.’ Keep your body high in the water and keep your head pointed down toward the bottom of the pool. Float in the position for several seconds.  This will help you get used to the way your body should feel when you’re in the core position.  Grip a buoy between your legs if they tend to sink. Repeat until you feel confident that you know how to assume the core position. Don’t hold the position for longer than you can hold your breath.