Hi Voyaging,
Welcome. For me, i simply place my bottles in a dark, dry, cool area to steep. I leave the cap on so no bacteria floats into the bottles and place them into my nightstand drawer away from sunlight and heat. In a week, usually my sweeteners and certain strong flavours mellow out. Some on here place them in the refrigerator. Some give the bottles a warm bath: fill the sink or small basin with warm water halfway, open the bottle caps, then dip the bottles into the warm water for a while.
These are the methods i know of. I know there are more lucrative ways but feel free to search the forums on the top right search bar. Im sure you will find a method that will suit you or you can follow mine.