The dough is quite buttery, so there's no need to flatten the snickerdoodles before putting them in the oven. The dough will naturally melt and flatten out as it bakes. If your snickerdoodles aren't flattening out, you may have used too much flour.  Be sure to use a kitchen scale (affiliate) to measure your flour accurately.  If your cookies are flattening out too much, your oven may not be calibrated (baking at a lower temperature than necessary), or you may not have used enough flour.