Since the Soleus is similar to the Gastrocnemius, we can do the same stretches, with a little variation.
This stretch is done using a wall for support. Standing close to the wall, place one foot back and one slightly forward. Your feet should be flat on the floor. Then squat down a little so that both knees are bent. Gently lean your body forward, towards the wall, until you feel a stretch in your lower leg. Hold for 10-15 seconds then switch sides.
If you want to do the step stretch, like the Gastrocnemius, then the slight variation is just bending your knees. If you want to do one leg at a time, stand with both feet on the step, then place one foot back, so that the heel can drop down. When you drop the heel down, bend both knees, and gently stretch the calf. Hold for 10-15 seconds then step down and repeat for the other leg.
If you want to do the step stretch with both legs at the same time, be sure that you keep both knees bent and the balls of the feet on the edge of the step. Drop your heels down slowly and feel the stretch in the lower calf. Hold for 10-15 seconds then relax.