If you have rough areas of sand/cement, you can do one of two things.
First, if its a large area of rough finish, you can buy some vinyl patching compound at Home Depot and 'float' the compound with a 10" puddy knife or trowel until smooth.
Second, if the area of concern is more of a "bump" or "nipple", you can buy a concrete sanding block at Home Depot and lightly rub the area until smooth.
Sometimes after sanding large areas of rough sand/cement, the sanding block will pull the larger pieces of sand or aggregate out of the pool base and create a small void. Just use the vinyl patching compound to float over these voids.