Tips:

I'd sand it with the buttpad on, otherwise you might be disappointed if the pad is too big when the stock's all finished.

I'd always use a sanding block wherever possible (hard or flexible, depending on which bit of the stock), otherwise the surface will end up wavey.

I'd mask up the chequering to avoid sanding off the points.

As the stock is beech (HW77?), I'd consider finishing it with coloured lacquer ... for one thing, you'll probably never get all of those dark patches out, and dark-ish lacquer will cover them up. Saving you a lot of sanding work... and it'll look much more like a factory finish.