The CDC requires that each mask must have a pocket on the backside of the mask for a filter to be placed inside of the mask. Do not let this intimidate you, it does not make the mask harder to make. You can either leave the pocket empty, provide a filter or sew the filter between the outer fabric of the mask, also refereed to as the interface. You are not required to provide the filter inside the pocket, but you can if you would like to.
Each template I provide on this website has a pocket built into the mask so you will not have to worry about figuring it out for yourself.
With a face mask
Without a face mask