Ann, If you are working on the outside of a mold you would push the stones into the walls after you have the mold completely covered. Wall has to be thick enough so stones don't go through to the inside of the planter. Use smaller stones towards the base of the planter. If you use larger ones they can stick out too far and make the planter sit lopsided. You will have to adjust the stones and the mix often because they keep shifting if you're working on the outside.
If you work on the inside of a mold the stones will be more embedded though you can carve out around the stones while it's still damp.
You can also make a plain sided planter and add stones after the fact by making up a mortar mix and mortaring the stones in place. If the planter isn't fresh made, submerge it in a bucket of water and let the water sink in to the piece before adding the mortar and stones. You need a damp surface to add any cement mix to an already cured piece.
If you are going to make your stones shiny use an outdoor clear varnish/poly and only paint the stones not the cement mix. I don't use varnish/poly so can't recommend one. Go to hardware/home improvement/masonry store or search online for stone/rock sealer.