With the brush in hand...the process is pretty simple.
The stain I'm working with here is DEFY Extreme Wood Stain. This water-based stain goes on easy and holds up well against UV damage.
The first step is to cut in the areas with a smaller hand brush that you can't reach with the push-brush like under the rails. Once everything is cut in, take the push-brush and just dip and brush. Work in sections and work from one end of the board all the way to other end. It's really just like sweeping or mopping your deck, but instead of a broom or a mop, you're using a brush. Be sure to avoid stopping or breaking in the middle of the boards as that can result in ugly lap marks.
The results look great. Not only are you getting your project done quickly, but a push-brush does a great job of really forcing the stain into the wood. That means that your staining job will not only look good, but it will also last longer.