The first step is pressure washing. <a href="https://www.house-painting-info.com/pressure-washing-stained-siding.html">Pressure washing stained siding</a> is a little different than washing painted siding. This is the most important step.
Most likely your old stain will come right off revealing all new wood. After the washing, and ample dry time, it is simply a matter of applying the new stain.