We all begin our martial arts journey at different points in our life and with a unique body situation. Age, experience, health, and possible past injuries are all a part of being a human being. Junan taiso is the starting point with learning and then expanding your capability. Think of it as a health method for preventing injury and expanding that capacity over time.
Begin your stretching routine by loosening up- go for a walk or move around a bit. The idea is to get your blood flowing and start limbering up. From there start from the top and work your way down loosening up your major joins and pivot points- neck, shoulders, arms/wrists, hips, legs, knees, and ankles. Pay special attention to the joints that our techniques attack and work on keeping them loose and limber as this will help with you nagare (flow) later in training.
Throughout your training, and it will change over time and at different points, you want to keep track of where you are tense or not as limber- slowly work on these areas to increase your range of flexibility. Also, pay special attention to your hips, wrists, and shoulders as there are points often rotated/used in class.
After your stretching routine your body should be in a relaxed and warmed up state. Take a few seconds to exist in the moment and feel what your body is like in this state- natural relaxation is important to good taijutsu, and later as you practice in class/at home this is the state of body that you want to try and remain in.
Keep in mind that there is no “rush” to get limber! Over the course of your martial arts journey you will have months and years to slowly improve yourself!