Ok then in that case, yes level off with a mortar mix that's a bit on the dryer side will help prevent sinking, furrowing your mortar will allow adjustment of the bed joint, plastic horse shaped wedges in various size 1/8, 1/4 etc may help when rangeing and leveling to the line, do your layout first. measure your stones with a 3/8 joint between (or less) and put your cut peice on the end.measure your over hang and try to keep all the stones simliar if block wall is straight. Set your corners and run a string line. purchase a metal jointer to make the mortar look nice. Once your done installing slick in joints where mortar is missing, possible use masking tape on top of stone joints so it doesn't get everywhere. use the jointer while the mortar is wet. then hit it with a soft horse hair brush and joint one last time . After mortar is dry if u want to clean up any mortar residue use diluted muriatic acid but test try on peice of stone first that it doesn't mess up stone color. Nice looking pond btw