Cover your work area with a large tarpaulin or sheet and put on a face mask. When you apply the sandpaper to the stone or start to pick at the stone, you will get a lot of flaking. In addition, sandstone contains silica, which can cause respiratory problems.
Draw a sketch of your sculpture idea on a piece of paper, preferably from at least two different views. Having a visual idea of what you want to make will help when it comes to shaping and refining the sandstone piece.
Use sandpaper to change the shape of the piece of sandstone before starting refining touches. Any cutting that you do with a carving tool suited for a harder rock will take away too much in flaking for you to retain a fine shape, so if you have a specific shape in mind, use sandpaper to wear away edges and corners interfering with that shape.
Mark specific spots for cuts with a pencil right on the sandstone. If you have certain lines you want to follow, go ahead and draw those on the surface.
Follow the lines you made with your leatherworking picks. Work at a slow but steady pace to reduce the chance of knocking off larger chunks with speed while also reducing the chance of losing your line by going too slowly.